zed_extension_api: Start a list of pending changes (#16305)

This PR starts a list of pending changes for the Zed extension API.

We'll want to keep this list updated as we note things that we want to
change in the next version of the extension API. This will help with
batching breaking changes together so that we're not constantly creating
new versions of the extension API for one-off changes.

Release Notes:

- N/A