Use Uri#path#fsPath in FileUri.fsPath()

[dhuebner]

Uri#path#fsPath() deals better with the case where backend and frontend are running on different OS.

See discussion here.

FileUri.fsPath() used to be exclusively used on the backend (I believe you moved it from node to common recently), where the only available OS was the backend OS. With that in mind, it was never an issue previously. Changing it to forward to Uri#path#fsPath would probably be a good idea though.

Originally posted by @msujew in #13601 (comment)

Feature Description:

FileUri.fsPath() should internally use Uri#path#fsPath() for conversion.