Support DidChangeWorkspaceFolders notifications
This PR enables the `WorkspaceFoldersServerCapabilities` capability for rust-analyzer and implemented support for the associated [`DidChangeWorkspaceFolders`](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#workspace_didChangeWorkspaceFolders) notification to allow clients to update the list of `workspaceFolders` sent during initialization.
## Motivation
This allows clients which lazily autodiscover their workspace roots (like the [helix editor](https://github.com/helix-editor/helix) once [my PR](https://github.com/helix-editor/helix/pull/5748) lands) avoid spawning multiple instances of RA. Right now such clients are forced to either:
* greedily discover all LSP roots in the workspace (precludes the ability to respond to new workspace roots)
* spawn multiple instance of rust-analyzer (one for each root)
* restart rust-analyzer whenever a new workspace is added
Some example use-cases are shown [here](https://github.com/helix-editor/helix/pull/5748#issuecomment-1421012523).
This PR will also improve support for VSCode (and Atom) multi workspaces.
## Implementation
The implementation was fairly straightforward as `rust-analyzer` already supports dynamically reloading workspaces, for example on configuration changes. Furthermore, rust-analyzer also already supports auto-discovering internal workspace from the `workspaceFolders` key in the initialization request. Therefore, the necessary logic just needed to be moved to a central place and reused.
3c89945e78