Refactor LanguageSelector to be disabled when snippet is locked

[solkaz]

Description

This PR refactors the LanguageSelector and SnippetAreaToolbar components to disable selecting a language when a snippet has been locked. The SnippetAreaToolbar will pass its readOnly prop to its child LanguageSelector, which will then forward that to its underlying SelectField element.

Motivation and Context

Fixes #395

Checklist:

• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.