keymap: Allow upper-case keys in keybinds (#27813)

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

crates/terminal/src/mappings/keys.rs

@@ -398,7 +398,7 @@ mod test {
                     false
                 ),
                 to_esc_str(
-                    &Keystroke::parse_case_insensitive(&format!("ctrl-{}", upper)).unwrap(),
+                    &Keystroke::parse(&format!("ctrl-{}", upper)).unwrap(),
                     &mode,
                     false
                 ),
@@ -415,7 +415,7 @@ mod test {
         for character in ascii_printable {
             assert_eq!(
                 to_esc_str(
-                    &Keystroke::parse_case_insensitive(&format!("alt-{}", character)).unwrap(),
+                    &Keystroke::parse(&format!("alt-{}", character)).unwrap(),
                     &TermMode::NONE,
                     true
                 )