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
