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.

assets/keymaps/vim.json

@@ -326,6 +326,22 @@
       "ctrl-o": "vim::TemporaryNormal"
     }
   },
+  {
+    "context": "vim_mode == helix_normal",
+    "bindings": {
+      "i": "vim::InsertBefore",
+      "a": "vim::InsertAfter",
+      "w": "vim::NextWordStart",
+      "e": "vim::NextWordEnd",
+      "b": "vim::PreviousWordStart",
+
+      "h": "vim::Left",
+      "j": "vim::Down",
+      "k": "vim::Up",
+      "l": "vim::Right"
+    }
+  },
+
   {
     "context": "vim_mode == insert && !(showing_code_actions || showing_completions)",
     "use_layout_keys": true,