Extract `InlineCompletionProvider` to its own crate (#20935)

This PR extracts the `InlineCompletionProvider` trait and its related
types out of `editor` and into a new `inline_completion` crate.

By doing so we're able to remove a dependency on `editor` from the
`copilot` and `supermaven` crates.

We did have to move `editor::Direction` into the `inline_completion`
crate, as it is referenced by the `InlineCompletionProvider`. This
should find a better home, at some point.

Release Notes:

- N/A