feat: implement edit variant that adds to current fold

[mitchellwrosen]

Overview

This PR adds a variant of edit (codename edit2) that adds definitions to the top of the file without a fold line below them. It's also careful not to add definitions that already exist in the latest typechecked file.

Test coverage

I've tested this manually and added a transcript.

Loose ends

Rename edit2 to ?

[aryairani]

Let's have edit.new be the existing behavior of adding the fold, edit be the new behavior that does not add a fold, like you'd originally suggested.

[mitchellwrosen]

@aryairani this is ready to go