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/editor/src/test/editor_test_context.rs

@@ -240,7 +240,7 @@ impl EditorTestContext {
     // unlike cx.simulate_keystrokes(), this does not run_until_parked
     // so you can use it to test detailed timing
     pub fn simulate_keystroke(&mut self, keystroke_text: &str) {
-        let keystroke = Keystroke::parse_case_insensitive(keystroke_text).unwrap();
+        let keystroke = Keystroke::parse(keystroke_text).unwrap();
         self.cx.dispatch_keystroke(self.window, keystroke);
     }