[StyleGuide] Allow let in JavaScript code

docs/style-guide.md

 # Catapult Style guide
 
 ## Base style guide
 
 Unless stated below, we follow the conventions listed in the [Chromium style
 guide](https://www.chromium.org/developers/coding-style) and [Google JavaScript
 style guide](http://google.github.io/styleguide/javascriptguide.xml).
 
 ## JavaScript
 
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 **Use of ES6 features is prohibited unless explicitly approved in the table below.** However, we're currently working to allow them.
 
 | Feature                                                                                                                                     | Status                                                                          |
 |---------------------------------------------------------------------------------------------------------------------------------------------|---------------------------------------------------------------------------------|
 | [Arrows](https://github.com/lukehoban/es6features#arrows)                                                                                   | [Approved](https://github.com/catapult-project/catapult/issues/2165)            |
 | [Classes](https://github.com/lukehoban/es6features#classes)                                                                                 | [Approved](https://github.com/catapult-project/catapult/issues/2176)            |
 | [Enhanced object literals](https://github.com/lukehoban/es6features#enhanced-object-literals)                                               | Approved                                                                        |
 | [Template strings](https://github.com/lukehoban/es6features#template-strings)                                                               | Approved                                                                        |
 | [Destructuring](https://github.com/lukehoban/es6features#destructuring)                                                                     | Approved                                                                        |
 | [Default, rest, and spread](https://github.com/lukehoban/es6features#default--rest--spread)                                                 | To be discussed                                                                 |
-| [`let` and `const`](https://github.com/lukehoban/es6features#let--const)                                                                    | `const` Approved, `let` pending discussion
+| [`let` and `const`](https://github.com/lukehoban/es6features#let--const)                                                                    | Approved and required for new code
 | [Iterators and `for...of`](https://github.com/lukehoban/es6features#iterators--forof)                                                       | Approved                                                                        |
 | [Generators](https://github.com/lukehoban/es6features#generators)                                                                           | Approved                                                                        |
 | [Unicode](https://github.com/lukehoban/es6features#unicode)                                                                                 | To be discussed                                                                 |
 | [Modules](https://github.com/lukehoban/es6features#modules)                                                                                 | To be discussed                                                                 |
 | [Module loaders](https://github.com/lukehoban/es6features#module-loaders)                                                                   | To be discussed                                                                 |
 | [`Map`, `Set`, `WeakMap`, and `WeakSet`](https://github.com/lukehoban/es6features#map--set--weakmap--weakset)                               | Approved                                                                        |
 | [Proxies](https://github.com/lukehoban/es6features#proxies)                                                                                 | To be discussed                                                                 |
 | [Symbols](https://github.com/lukehoban/es6features#symbols)                                                                                 | To be discussed                                                                 |
 | [Subclassable Built-ins](https://github.com/lukehoban/es6features#subclassable-built-ins)                                                   | Approved                                                                        |
 | [Promises](https://github.com/lukehoban/es6features#promises)                                                                               | Approved                                                                        |
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   1. Emit a clear warning and continue if the error is non-catastrophic
   2. Fail loudly if the error is catastrophic
 
 If fixing the problem is hard but a simple workaround is possible, then using
 the workaround is OK so long as:
 
   1. An issue is created to track the problem.
   2. The defensive patch is wrapped in a `// TODO` linking to that issue.
   3. The todo and defensive patch are removed after the problem is fixed.