Edit: Show line-level progress decorations for ongoing edits

[umpox]

What:

• Currently we show live decorations of the character-level diff that we calculate for Edits.
• This can appear quite confusing for users, also partly glitchy because the character-level diff is greedy and picks up characters from later on in the selection, even though they will soon be included in the LLM’s response.

How:

• Instead we should show line-level decorations for ongoing edits
• We should include:
  • The current active line
  • Previously visited lines
  • Remaining lines
• We could also possibly include “changed lines” to show where Cody is making edits before we apply that edit.