Browser crashes editing button template

[twschiller]

Describe the bug

• Browser crashes editing button HTML template
• https://pixiebrix.slack.com/archives/C0436P48QHY/p1717017455901409 (includes Loom)

Definition of Done

• Must have: If the HTML is empty, the starter brick should not insert anything
• Must have: If the HTML is invalid, the starter brick should not insert anything
• Nice to have: add analysis rule that shows error on invalid HTML template. (IIRC, the value should be a valid HTML element even though it's a mustache template)

Discussion

• DOMPurify might fix some invalid HTML automatically

Workaround

• Likely workaround is to only provide valid HTML to for the template

Related Code

• https://github.com/pixiebrix/pixiebrix-extension/blob/f4fcceca65eca7e3c99095b5cda7a55a87a27a16/src/starterBricks/menuItem/menuItemExtension.ts#L554-L554
• Related item: https://github.com/pixiebrix/pixiebrix-extension/blob/f4fcceca65eca7e3c99095b5cda7a55a87a27a16/src/starterBricks/menuItem/menuItemExtension.ts#L912-L912