Restructure assistant edits to show all changes in a proposed-change editor (#18240)

This changes the `/workflow` command so that instead of emitting edits
in separate steps, the user is presented with a single tab, with an
editable diff that they can apply to the buffer.

Todo

* Assistant panel
* [x] Show a patch title and a list of changed files in a block
decoration
* [x] Don't store resolved patches as state on Context. Resolve on
demand.
    * [ ] Better presentation of patches in the panel
    * [ ] Show a spinner while patch is streaming in
* Patches
* [x] Preserve leading whitespace in new text, auto-indent insertions
    * [x] Ensure patch title is very short, to fit better in tab
* [x] Improve patch location resolution, prefer skipping whitespace over
skipping `}`
    * [x] Ensure patch edits are auto-indented properly
* [ ] Apply `Update` edits via a diff between the old and new text, to
get fine-grained edits.
* Proposed changes editor
    * [x] Show patch title in the tab
    * [x] Add a toolbar with an "Apply all" button
* [x] Make `open excerpts` open the corresponding location in the base
buffer (https://github.com/zed-industries/zed/pull/18591)
* [x] Add an apply button above every hunk
(https://github.com/zed-industries/zed/pull/18592)
* [x] Expand all diff hunks by default
(https://github.com/zed-industries/zed/pull/18598)
    * [x] Fix https://github.com/zed-industries/zed/issues/18589
* [x] Syntax highlighting doesn't work until the buffer is edited
(https://github.com/zed-industries/zed/pull/18648)
* [x] Disable LSP interaction in Proposed Changes editor
(https://github.com/zed-industries/zed/pull/18945)
* [x] No auto-indent? (https://github.com/zed-industries/zed/pull/18984)
* Prompt
    * [ ] make sure old_text is unique

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>
Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Richard <richard@zed.dev>
Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Nate Butler <iamnbutler@gmail.com>
Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Richard Feldman <oss@rtfeldman.com>

crates/text/src/text.rs

@@ -1427,7 +1427,7 @@ impl Buffer {
     fn undo_or_redo(&mut self, transaction: Transaction) -> Operation {
         let mut counts = HashMap::default();
         for edit_id in transaction.edit_ids {
-            counts.insert(edit_id, self.undo_map.undo_count(edit_id) + 1);
+            counts.insert(edit_id, self.undo_map.undo_count(edit_id).saturating_add(1));
         }
 
         let operation = self.undo_operations(counts);