Rework diff rendering to allow putting the cursor into deleted text, soft-wrapping and scrolling deleted text correctly (#22994)

Closes #12553

* [x] Fix `diff_hunk_before`
* [x] Fix failure to show deleted text when expanding hunk w/ cursor on
second line of the hunk
* [x] Failure to expand diff hunk below the cursor.
* [x] Delete the whole file, and expand the diff. Backspace over the
deleted hunk, panic!
* [x] Go-to-line now counts the diff hunks, but it should not
* [x] backspace at the beginning of a deleted hunk deletes too much text
* [x] Indent guides are rendered incorrectly 
* [ ] Fix randomized multi buffer tests

Maybe:
* [ ] Buffer search should include deleted text (in vim mode it turns
out I use `/x` all the time to jump to the next x I can see).
* [ ] vim: should refuse to switch into insert mode if selection is
fully within a diff.
* [ ] vim `o` command when cursor is on last line of deleted hunk.
* [ ] vim `shift-o` on first line of deleted hunk moves cursor but
doesn't insert line
* [x] `enter` at end of diff hunk inserts a new line but doesn't move
cursor
* [x] (`shift-enter` at start of diff hunk does nothing)
* [ ] Inserting a line just before an expanded hunk collapses it

Release Notes:


- Improved diff rendering, allowing you to navigate with your cursor
inside of deleted text in diff hunks.

---------

Co-authored-by: Conrad <conrad@zed.dev>
Co-authored-by: Cole <cole@zed.dev>
Co-authored-by: Mikayla <mikayla@zed.dev>
Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>
Co-authored-by: Michael <michael@zed.dev>
Co-authored-by: Agus <agus@zed.dev>
Co-authored-by: João <joao@zed.dev>

crates/workspace/src/workspace.rs

@@ -102,6 +102,8 @@ use crate::persistence::{
     SerializedAxis,
 };
 
+pub const SERIALIZATION_THROTTLE_TIME: Duration = Duration::from_millis(200);
+
 static ZED_WINDOW_SIZE: LazyLock<Option<Size<Pixels>>> = LazyLock::new(|| {
     env::var("ZED_WINDOW_SIZE")
         .ok()
@@ -4344,7 +4346,6 @@ impl Workspace {
         cx: &mut AsyncWindowContext,
     ) -> Result<()> {
         const CHUNK_SIZE: usize = 200;
-        const THROTTLE_TIME: Duration = Duration::from_millis(200);
 
         let mut serializable_items = items_rx.ready_chunks(CHUNK_SIZE);
 
@@ -4369,7 +4370,9 @@ impl Workspace {
                 }
             }
 
-            cx.background_executor().timer(THROTTLE_TIME).await;
+            cx.background_executor()
+                .timer(SERIALIZATION_THROTTLE_TIME)
+                .await;
         }
 
         Ok(())