Correct other end visual block functionality (#27678)

Closes #27385

Builds on #27604 so that `vim::OtherEnd` works in visual block mode.
This is accomplished by reversing the order of active selections in the
buffer when the user hit `o`, so that the cursor moves diagonally across
the selection. The current behavior is preserved for `shift-o`, which is
how the cursors behave in vim.

We'll close #27604 since this encapsulates that change, but if you'd
prefer to take only the visual block motion component, we'll keep the
branch for #27604 open.

Test case: growing a box down and to the right, other ending, followed
by growing and shrinking the box:


https://github.com/user-attachments/assets/1df544e1-efce-4354-b354-bbfec007a7df

Test case: growing a box up and to the left, other ending, followed by
growing and shrinking the box:


https://github.com/user-attachments/assets/2f6d7729-c63a-4486-960b-23474c2e507a



Release Notes:
- Improved visual block mode when cursor is at beginning of selection
- Improved visual block mode so that `o` and `shift-o` reach parity with
vim

---------

Co-authored-by: KyleBarton <kjbarton4@gmail.com>

crates/vim/README.md

@@ -28,7 +28,7 @@ but while developing this test you'll need to run it with the neovim flag enable
 cargo test -p vim --features neovim test_visual_star_hash
 ```
 
-This will run your keystrokes against a headless neovim and cache the results in the test_data directory.
+This will run your keystrokes against a headless neovim and cache the results in the test_data directory. Note that neovim must be installed and reachable on your $PATH in order to run the feature.
 
 
 ## Testing zed-only behavior