Support dev servers defined in a mojoconfig file.

mojoconfig

+# This is a configuration file for devtools (`mojo_run`, `mojo_test) running
+# within a Mojo checkout.
+
+# The content has to parse to a Python dictionary literal. Strings of the form
+# '@{ABC}' are aliases that will be substituted for their values before
+# evaluation:
+#  '@{BUILD_DIR}': path to the output directory
+
+{
+  # Each dev server will be configured as specified and mapped for the
+  # indicated host using --map-origin.
+  'dev_servers': [
+    {
+      'host': 'https://core.mojoapps.io/',
+      # First matching prefix will apply. Within the directiories specified for
+      # a prefix, first location that contains the requested path will apply.
+      'mappings': [
+        ('packages/', ['@{BUILD_DIR}/gen/dart-pkg/packages']),
+        ('', ['@{BUILD_DIR}']),
+      ],
+    },
+  ],
+}