This PR adds cross-file Typescript support to vscode-web. Like the desktop version, it starts two tsservers, one in syntax mode and one in semantic mode. This means that you can now import from from other files without having them open in the editor.

It does this by changing the web-based tsserver host, which now depends on vscode-wasm to communicate with a virtual file system. The code is substantially similar to before, but supports slightly more of the tsserver API, and adds a translation layer for talking to a virtual file system based on URIs instead of a real one based on paths. It also has adapter code for responding to watch events asynchronously but communicating the event to tsserver sychronously.

It makes some important changes to the typescript extension:

1. WorkerServerProcess in serverProcess.browser.ts now imports vscode-wasm packages and sets up several MessageChannels with the tsserver host.
2. Cancellation support is added in ProcessBasedTsServer in src/tsServer/server.ts.
3. WorkerServerProcess passes the extensionURI to the tsserver host so that lib*d.ts files can be redirected to their CDN location (the code is copied from typescriptServiceClient.toResource).

I don't know if any of these are done in the right place, or in the right way. I also have some questions about the host implementation:

For @sheetalkamat:

• require -- is this needed? In TS, it's only used in project system. Answer: No, only needed for node environments.
• trace -- is this needed? In TS, it's only used in project system. Answer: No.
• useCaseSensitiveFileNames -- old version says 'false' is the safest option, but the virtual fs is case sensitive. Is false still better? Answer: match the virtual fs: case-sensitive.

For @jrieken:

• the node code manually skips '.' and '..' in the array returned by fs.readdirSync. Will readDirectory ever return '.' and '..'?
• writeOutputIsTTY -- I'm using apiClient.vscode.terminal.write -- is it a tty?
• getWidthOfTerminal -- I don't know where to find this on apiClient.vscode.terminal either
• clearScreen -- the node version of the code writes \x1BC to the terminal. Would this work for vscode?
• readFile/writeFile -- the node version handles utf8, utf16le and manually converts big-endian to utf16 little-endian. How does the in-memory filesystem handle this? There's no place to specify encoding.
• realpath/getDirectories/readDirectory -- how do I resolve symlinks? I copied some fs.statSync-based code for getDirectories, but I don't think it's right.

Not sure who will know these:

• resolvePath -- node version uses path.resolve. Is it OK to use that? Or should I re-implement it? Just use identity like the old web code?
• createSHA256Hash -- the browser version is async, so I skipped it. Is this OK?

This PR still needs some cleanup, which I will do next:

• cancellation should only retain one cancellation checker
  • the one that matches the current request id
  • but that means tracking (or retrieving from tsserver) the request id (aka seq?)
  • and correctly setting/resetting it on the cancellation token too.
• Cancellation code in vscode is suspiciously prototypey.
  • Specifically, it adds the vscode-wasm cancellation to original cancellation code, but should actually switch to the former for web only.
  • looks like isWeb() is a way to check for being on the web
• create multiple watchers
  • on-demand instead of watching everything and checking on watch firing
• Need to parse and pass args through so that the syntax server isn't hard-coded to actually be another semantic server
• clear out TODOs
• think about implementing all the other ServerHost methods
  • also copy details from previous implementation (although it's syntax-only so only covers part)
• organise webServer.ts into multiple files (in a separate PR, to minimise churn)
• add semicolons everywhere; vscode's lint doesn't seem to complain, but the code clearly uses them

These are tracked in the PR itself in extensions/typescript-language-fetaures/web/README.md