Include buildfiles in VFS
We subscribe to `textDocument/didSave` for `filesToWatch`, but the VFS doesn't contain those files. Before https://github.com/rust-lang/rust-analyzer/pull/18105, this would bring down the server. Now, it's only a benign error logged:
```
ERROR notification handler failed handler=textDocument/didSave error=file not found: /foo/bar/TARGETS
```
It's benign, because we will also receive a `workspace/didChangeWatchedFiles` for the file which will invalidate and load it.
Explicitly include the buildfiles in the VFS to prevent the handler from erroring.
546339a7be