Make shifted letter bindings robust when terminals report uppercase
letters without explicit SHIFT modifiers.
This updates keybinding matching so bindings like `shift-i`, `shift-a`,
and `shift-o` also match `I`, `A`, and `O` event forms, and adds
regression tests covering Vim normal-mode actions for line-start insert,
line-end append, and open-line-above with shift-only bindings.
eb2275a64a