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.

assets/keymaps/linux/jetbrains.json

@@ -58,7 +58,8 @@
       "ctrl-shift-home": "editor::SelectToBeginning",
       "ctrl-shift-end": "editor::SelectToEnd",
       "ctrl-f8": "editor::ToggleBreakpoint",
-      "ctrl-shift-f8": "editor::EditLogBreakpoint"
+      "ctrl-shift-f8": "editor::EditLogBreakpoint",
+      "ctrl-shift-u": "editor::ToggleCase"
     }
   },
   {