LSPs can offer a rename capability to rename a "symbol" across a workspace.
We're not there yet, but this is a starting point. In VS Code it can be used by
right clicking a symbol (currently variable or call to local function) and
selecting "Rename symbol". A dialog should pop up asking for a new name.

Official spec:
https://microsoft.github.io/language-server-protocol/specifications/specification-current/#textDocument_rename

- Make rename_example a `.ex` file so we can later test multi-file
  refactors
- Simplify rename_example to make it easier to follow
- Extract some common patterns out in the tests