GUI actions to do simple edits

[Bzero]

Typstwriter should support to do the most simple edits of a typst document using the GUI.

The following would be useful simple edits:

• Make text bold
• Make text italic
• Underline text
• Quote text
• Insert heading (use a drop-down menu to select level)
• Insert code block
• Insert math block
• Insert markdown block
• Insert label
• Insert reference
• Insert link
• Start numbered list
• Start bullet list
• Insert image (or figure)