file watcher: avoid fs.watch

nodejs `fs.watch` is unreliable, see https://github.com/aws/aws-toolkit-vscode/pull/2381

- From https://github.com/paulmillr/chokidar#why :
> Node.js fs.watch ... Emits most changes as rename.
- os/platform-specific quirks

~~The reason we use fs.watch for cases like https://github.com/aws/aws-toolkit-vscode/pull/2381/files is because vscode's builtin watcher only works for files in a workspace folder. https://github.com/microsoft/vscode/issues/3025~~

- Update: vscode watcher now supports out-of-workspace paths. https://github.com/microsoft/vscode/issues/3025#issuecomment-1011815376
- Update (2023): vscode proposal for a new `createFileSystemWatcher` API https://github.com/microsoft/vscode/issues/169724

## Proposal

- ~~use https://github.com/paulmillr/chokidar or https://www.npmjs.com/package/fb-watchman instead of `fs.watch`~~
- Use vscode native support https://github.com/microsoft/vscode/issues/3025
- potential future advantage: this would presumably also work with the vscode virtual fs (VFS) layer #2205 #1823

Start by locating the current fs.watch call sites related to pull request #2381, then read the VS Code native watcher discussion in issue #3025 and the newer createFileSystemWatcher proposal. Done means replacing the unreliable watcher use with VS Code native support for the affected out-of-workspace paths and verifying the relevant file-change behavior.

Written by the indexing model from the issue text.