Add toggle case command (#28415)

A small addition for those coming from JetBrain's IDEs. A behavioral
detail: when any upper case character is detected, the command defaults
to toggling to lower case.

> Note that when you apply the toggle case action to the CamelCase name
format, IntelliJ IDEA converts the name to the lower case.


https://www.jetbrains.com/help/idea/working-with-source-code.html#edit_code_fragments

Release Notes:

- Added an `editor: toggle case` command. Use `cmd-shift-u` for macOS
and `ctrl-shift-u` for Linux, when using the `JetBrains` keymap.

crates/editor/src/actions.rs

@@ -420,6 +420,7 @@ actions!(
         Tab,
         Backtab,
         ToggleBreakpoint,
+        ToggleCase,
         DisableBreakpoint,
         EnableBreakpoint,
         EditLogBreakpoint,