vim: Support keybinding z. (#8702)

`z.` is similar to zz but moves the cursor to the first non-blank
character.

From the documentation:
```
z.   Redraw, line [count] at center of window (default cursor line). Put cursor at first non-blank in the line.
zz   Like "z.", but leave the cursor in the same column.
```

Release Notes:

- Support the `z.` vim keybinding: Center cursor in window and put
cursor at first non-blank

assets/keymaps/vim.json

@@ -218,6 +218,7 @@
       // z commands
       "z t": "editor::ScrollCursorTop",
       "z z": "editor::ScrollCursorCenter",
+      "z .": ["workspace::SendKeystrokes", "z z ^"],
       "z b": "editor::ScrollCursorBottom",
       "z c": "editor::Fold",
       "z o": "editor::UnfoldLines",