Rename extension API method, provide interface
Rename extension API method, provide interface

- Rename `register_diff_provider` --> `registerDiffProvider`
- Ensure `IGitExtension` is listed in `provides` of plugin.
- Re-order declarations to enable previous (and help reading of code).

jupyterlab/jupyterlab-git,jupyterlab/jupyterlab-git,jupyterlab/jupyterlab-git