- Pass the file path of the buffer which triggered the server to start
  to decide the first workspace root. In the case of a server restart,
  remember the file that was open when the restart command was issued.
- Add a `lsc#workspace#findMarker` utility function to search upwards
  from a file to a directory which contains some other file that marks
  the "root" for that project.

TODO:
- Add docs.
- Check capabilities before sending the message.
- Figure out if removing roots is necessary. This would require tracking
  per-server lists of files.
- Allow giving a name to the roots.
- Implement the `workspace/workspaceFolders` request.

Potential future support
- A known file like `.lsc_root` to reliably locally mark the root for
  any given workspace.
- Support for user specific project roots, or perhaps just support the
  common ones from the start, like `.git/`

- Add a stub for `lsc#workspace#byMarker()` to make it easier to write
  the tests.
- Read client capabilities in `StubServer`.
- Outline some tests. Add a test reading client capabilities.

Report stderr for vim.expr (might be helpful in other places too). Fix
the bug surfaced by this output with an extra closing paren.

Send an extra "<cr>" since the multiple sendKeys seem to leave vim in a
state with a "Press Enter to continue", maybe where it prints the
function that was just created?