Custom on_enter LSP method

[sdankel]

Description

Closes #4728

Moves the on_enter capabilities to a custom LSP method that simply returns the edits to the client, rather than directly applying the edits as part of did_change.

With this approach, there is still some lag if you hit enter many times, but it seems to be working better than before.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.