Edit: Fix race condition on save (intended deletions vs formatter)

[umpox]

Description

event.waitUntil on onWillSaveTextDocument supports taking a void promise but it does not seem to reliably wait for it.

event.waitUntil also supports taking a promise of TextEdit[]. It seems it will guarantee the lines will be deleted before the save with this approach/

Test plan

1. Create many edits
2. Save them
3. Check there's no race conditions between formatters and our manual line deletion