Move mojo edk into mojo/edk

mojo/embedder/README.md

-Mojo Embedder API
-=================
-
-The Mojo Embedder API is an unstable, internal API to the Mojo system
-implementation. It should be used by code running on top of the system-level
-APIs to set up the Mojo environment (instead of directly instantiating things
-from src/mojo/system).
-
-Example uses: Mojo shell, to set up the Mojo environment for Mojo apps; Chromium
-code, to set up the Mojo IPC system for use between processes. Note that most
-code should use the Mojo Public API (under src/mojo/public) instead. The
-Embedder API should only be used to initialize the environment, set up the
-initial MessagePipe between two processes, etc.