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/common/connection.ts

@@ -25,6 +25,8 @@ export interface InitData {
 	readonly shell: string;
 	readonly extensionsDirectory: string;
 	readonly builtInExtensionsDirectory: string;
+	readonly extraExtensionDirectories: string[];
+	readonly extraBuiltinExtensionDirectories: string[];
 }
 
 export interface SharedProcessData {