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/test/server.test.ts

@@ -3,9 +3,11 @@ import { createClient } from "./helpers";
 describe("Server", () => {
 	const dataDirectory = "/tmp/example";
 	const workingDirectory = "/working/dir";
+	const extensionsDirectory = "/tmp/example";
 	const builtInExtensionsDirectory = "/tmp/example";
 	const cacheDirectory = "/tmp/cache";
 	const client = createClient({
+		extensionsDirectory,
 		builtInExtensionsDirectory,
 		cacheDirectory,
 		dataDirectory,