feat: Add key bindings for uppercase letters for bold, italic and underline (#2478)

This way, key bindings 'Mod-B', 'Mod-I' and 'Mod-U' with active caps
lock have the same effect as their lowercase siblings.

Prosemirror examples did the same, see ProseMirror/prosemirror#895

Fixes: #2426

Signed-off-by: Jonas <jonas@freesources.org>

packages/extension-underline/src/underline.ts

@@ -66,6 +66,7 @@ export const Underline = Mark.create<UnderlineOptions>({
   addKeyboardShortcuts() {
     return {
       'Mod-u': () => this.editor.commands.toggleUnderline(),
+      'Mod-U': () => this.editor.commands.toggleUnderline(),
     }
   },
 })