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/test_data/test_visual_block_mode_down_right.json

@@ -0,0 +1,9 @@
+{"Put":{"state":"The ˇquick brown\nfox jumps over\nthe lazy dog"}}
+{"Key":"ctrl-v"}
+{"Key":"l"}
+{"Key":"l"}
+{"Key":"l"}
+{"Key":"l"}
+{"Key":"l"}
+{"Key":"j"}
+{"Get":{"state":"The «quick ˇ»brown\nfox «jumps ˇ»over\nthe lazy dog","mode":"VisualBlock"}}

crates/vim/test_data/test_visual_block_mode_other_end.json

@@ -0,0 +1,11 @@
+{"Put":{"state":"The quick brown\nfox jˇumps over\nthe lazy dog"}}
+{"Key":"ctrl-v"}
+{"Key":"l"}
+{"Key":"l"}
+{"Key":"l"}
+{"Key":"l"}
+{"Key":"j"}
+{"Get":{"state":"The quick brown\nfox j«umps ˇ»over\nthe l«azy dˇ»og","mode":"VisualBlock"}}
+{"Key":"o"}
+{"Key":"k"}
+{"Get":{"state":"The q«ˇuick »brown\nfox j«ˇumps »over\nthe l«ˇazy d»og","mode":"VisualBlock"}}

crates/vim/test_data/test_visual_block_mode_shift_other_end.json

@@ -0,0 +1,11 @@
+{"Put":{"state":"The quick brown\nfox jˇumps over\nthe lazy dog"}}
+{"Key":"ctrl-v"}
+{"Key":"l"}
+{"Key":"l"}
+{"Key":"l"}
+{"Key":"l"}
+{"Key":"j"}
+{"Get":{"state":"The quick brown\nfox j«umps ˇ»over\nthe l«azy dˇ»og","mode":"VisualBlock"}}
+{"Key":"shift-o"}
+{"Key":"k"}
+{"Get":{"state":"The quick brown\nfox j«ˇumps »over\nthe lazy dog","mode":"VisualBlock"}}

crates/vim/test_data/test_visual_block_mode_up_left.json

@@ -0,0 +1,9 @@
+{"Put":{"state":"The quick brown\nfox jumpsˇ over\nthe lazy dog"}}
+{"Key":"ctrl-v"}
+{"Key":"h"}
+{"Key":"h"}
+{"Key":"h"}
+{"Key":"h"}
+{"Key":"h"}
+{"Key":"k"}
+{"Get":{"state":"The «ˇquick »brown\nfox «ˇjumps »over\nthe lazy dog","mode":"VisualBlock"}}