Reverts the error behavior introduced in #27558. Upper-case keys in
keybindings no longer generate errors, instead they are transformed into
`shift-{KEY}`
e.g. `ctrl-N` becomes `ctrl-shift-n`

The behavior introduced in #27558 where "special" keys such as function
keys, `control`, `shift`, etc. Are parsed case-insensitively is
preserved.

Release Notes:
- Improved how upper-case characters are handled in keybinds. "special"
keys such as the function keys, `control`, `shift`, etc. are now parsed
case-insensitively, so for example `F8`, `CTRL`, `SHIFT` are now
acceptable alternatives to `f8`, `ctrl`, and `shift` when declaring
keybindings. Additionally, upper-case (ascii) characters will now be
converted explicitly to `shift` + the lowercase version of the
character, to match the Vim behavior.
NOTE: Release notes above should replace the release notes from #27558