Extra extensions directories (#694)

* Allow setting paths for builtin exts and extra dirs

The extra directories aren't used yet, just available from the
environment service and to the shared process.

* Utilize extra builtin extensions path

* Utilize extra extensions directory

* Fix cached mtimes for extra extension dirs

* Simplify extension cache equality check

packages/protocol/src/browser/client.ts

@@ -278,6 +278,8 @@ export class Client {
 					shell: init.getShell(),
 					extensionsDirectory: init.getExtensionsDirectory(),
 					builtInExtensionsDirectory: init.getBuiltinExtensionsDir(),
+					extraExtensionDirectories: init.getExtraExtensionDirectoriesList(),
+					extraBuiltinExtensionDirectories: init.getExtraBuiltinExtensionDirectoriesList(),
 				};
 				this.initDataEmitter.emit(this._initData);
 				break;