Implement rich text editing

[stackotter]

Pretty self explanatory follow on from #149. This will be quite a tricky one. Rich text editing features differ significantly across platforms. Identifying the set of features supported by all backends will be a good first step.

Backend implementations:

• AppKitBackend and UIKitBackend: Use NSTextView and UITextView. See this blog post for some internal details.
• WinUIBackend: Use RichEditBox
• GtkBackend: Use TextView. Styling can be applied to the underlying TextBuffer via TextTags. Tags also support setting layout properties such as text justification and letter spacing.
• Gtk3Backend: Use TextView. Seems similar to Gtk 4's TextView, but I'd assume that it's probably somewhat worse.