Implement Helix Support (WIP) (#19175)

Closes #4642 

- Added the ability to switch to helix normal mode, with an additional
helix visual mode.
- <kbd>ctrl</kbd><kbd>h</kbd> from Insert mode goes to Helix Normal
mode. <kbd> i </kbd> and <kbd> a </kbd> to go back.
- Need to find a way to perform the helix normal mode selection with
<kbd> w </kbd>, <kbd>e </kbd>, <kbd> b </kbd> as a first step. Need to
figure out how the mode will interoperate with the VIM mode as the new
additions are in the same crate.

crates/vim/src/test/neovim_connection.rs

@@ -442,6 +442,7 @@ impl NeovimConnection {
             }
             Mode::Insert | Mode::Normal | Mode::Replace => selections
                 .push(Point::new(selection_row, selection_col)..Point::new(cursor_row, cursor_col)),
+            Mode::HelixNormal => unreachable!(),
         }
 
         let ranges = encode_ranges(&text, &selections);