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

src/static/ribbit-core.css

@@ -55,3 +55,13 @@
 #ribbit.wysiwyg blockquote.ribbit-editing::before {
     content: "> ";
 }
+
+#ribbit.vim-normal {
+    cursor: default;
+    caret-color: transparent;
+    border-left: 3px solid #4af;
+}
+
+#ribbit.vim-insert {
+    border-left: 3px solid #4f4;
+}