Vim keybindings for source edit mode

VimHandler activates in source (edit) mode only. Two modes:
- Insert: standard typing, Esc enters normal mode
- Normal: vim navigation and editing, i/a/o/O enter insert

Normal mode commands:
  h/j/k/l: cursor movement
  w/b: word forward/back
  0/$: line start/end
  gg/G: document start/end
  i/a/o/O: enter insert mode
  x: delete char
  dd: delete line
  u: undo
  Ctrl+r: redo

test/setup.ts

@@ -22,7 +22,10 @@ export function getWindow(): any {
 }
 
 export function ribbit(): any {
-    return getWindow().ribbit;
+    const w = getWindow();
+    const r = w.ribbit;
+    r.window = w;
+    return r;
 }
 
 export function resetDOM(content = 'test'): void {