[api] Improve callback FS - #64447

Prior to #64115, we had

const api = new API({
  fs: createVirtualFileSystem({
    "/tsconfig.json": "{}"
  })
});

which worked by registering callbacks for readFile, fileExists, getAccessibleEntries, etc. for the server to call back to the client with.

One piece of feedback that surfaced with the callbacks was that if you supplied your own, rather than using createVirtualFileSystem, any callbacks you omitted silently resulted in the server using the OS file system for those callbacks, making it easy to create an inconsistency where you supply readFile but forget to supply fileExists.

Additionally, now that we have #64115, file system callbacks might still be useful, but they shouldn't be used to serve up a complete in-memory file system. Using #64115 accomplishes the same thing with much better performance, since the server avoids a round trip per FS call.

So, this PR

• removes createVirtualFileSystem (or rather, moves it to test utilities so we can keep relying on it for testing)
• changes the callback-based file system to enforce specifying an implementation for every function.

You don't have to implement a callback for every function—there are symbols that represent a few possible server-side implementations. For example, if I want to override readFile only, and let everything else use the OS, previously I would have specified only readFile. Now, that looks like this:

import { API } from "typescript/unstable/sync";
import { serverFS } from "typescript/unstable/fs";

const api = new API({
  fs: {
    readFile: customReadFileImplementation,
    fileExists: serverFS.useOS,
    directoryExists: serverFS.useOS,
    getAccessibleEntries: serverFS.useOS,
    stat: serverFS.useOS,
    realpath: serverFS.useOS,
    writeFile: serverFS.useOS,
  },
});

Every key is required, so if we ever expand the interface, you'll get notified with a type error. (serverFS.useOS is a unique symbol sentinel, not an actual callback.)

Other serverFS functions include

• every callback supports serverFS.error which panics (recovered, try/catchable from the client)—useful if you've set up a full in-memory snapshot file system and want to assert to yourself that you're never accidentally touching the real FS
• writeFile supports serverFS.noop
• stat supports serverFS.fakeStat (infers existence/mode from fileExists and directoryExists)
• realpath supports serverFS.identity (the identity function)

Those symbols can also be returned from callbacks to delegate control back to the server:

const api = new API({
  fs: {
    readFile: fileName => {
      if (shouldOverride(fileName)) {
        return overrideFileContents[fileName];
      }
      return serverFS.useOS;
    },
    // ...
  },
});