This fixes #11361 and #8764 by making sure we pass a directory as
`WorkspaceFolder` to the language server.

We already compute the `working_dir` correctly when
`self.root_path.is_file()`, but we didn't use it.

Release Notes:

- Fixed language servers (such as `gopls`) not starting up correctly
when opening a single file in Zed.
([#11361](https://github.com/zed-industries/zed/issues/11361) and
[#8764](https://github.com/zed-industries/zed/issues/8764)).