feat: add custom context menu rendering

[BeksOmega]

The basics

• I validated my changes

The details

Resolves

Fixes #6406

Proposed Changes

Fixes the types on context menu items so that you can pass your own HTML elements. The context menu already supports this, we were just restricting it with our documented types.

Reason for Changes

Peoples want to be able to add their own icons and whatnot to the context menu! This enables them to do so.

Test Coverage

Manually tested by changing the displayText() method on the duplicate option to the following:

    displayText() {
      const elem = document.createElement('span');
      elem.appendChild(document.createTextNode('test'));
      elem.style.fontWeight = 'bold';
      return elem;
    },

Documentation

https://developers.google.com/blockly/guides/configure/web/context-menus should be updated.

Additional Information

I looked into whether we need to do anything to support people using our CSS classes, but they're all applied to a wrapper object that the text/html element gets appended to, so I don't think there's anything we need to surface.