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/diagnostics/src/items.rs

@@ -1,11 +1,11 @@
 use std::time::Duration;
 
-use editor::{AnchorRangeExt, Editor};
+use editor::Editor;
 use gpui::{
     EventEmitter, IntoElement, ParentElement, Render, Styled, Subscription, Task, View,
     ViewContext, WeakView,
 };
-use language::{Diagnostic, DiagnosticEntry};
+use language::Diagnostic;
 use ui::{h_flex, prelude::*, Button, ButtonLike, Color, Icon, IconName, Label, Tooltip};
 use workspace::{item::ItemHandle, StatusItemView, ToolbarItemEvent, Workspace};
 
@@ -148,11 +148,7 @@ impl DiagnosticIndicator {
             (buffer, cursor_position)
         });
         let new_diagnostic = buffer
-            .diagnostics_in_range(cursor_position..cursor_position, false)
-            .map(|DiagnosticEntry { diagnostic, range }| DiagnosticEntry {
-                diagnostic,
-                range: range.to_offset(&buffer),
-            })
+            .diagnostics_in_range::<_, usize>(cursor_position..cursor_position)
             .filter(|entry| !entry.range.is_empty())
             .min_by_key(|entry| (entry.diagnostic.severity, entry.range.len()))
             .map(|entry| entry.diagnostic);