feat(developer): visual LDML editor prototype 🗼

[srl295]

Spec:

• spec(developer): LDML Keyboard Designer 🗼 #5016

Develop a visual editor for LDML.
Add a webview to the VSCode plugin that does a basic load of the XML file and displays some initial UI elements reflecting the contents of the keyboard.

Blocked by:

• feat(developer): VSCode Plugin for kpj/xml/kmn builds 🗼 #12756