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/vscode/src/fill/environmentService.ts

@@ -12,6 +12,18 @@ export class EnvironmentService extends environment.EnvironmentService {
 	public get extensionsPath(): string {
 		return paths.getExtensionsDirectory();
 	}
+
+	public get builtinExtensionsPath(): string {
+		return paths.getBuiltInExtensionsDirectory();
+	}
+
+	public get extraExtensionPaths(): string[] {
+		return paths.getExtraExtensionDirectories();
+	}
+
+	public get extraBuiltinExtensionPaths(): string[] {
+		return paths.getExtraBuiltinExtensionDirectories();
+	}
 }
 
 const target = environment as typeof environment;