Support MathML

[raquo]

I've seen an uptick in questions about MathML support recently fro some reason, and it turns out that we don't have proper support for it.

We support HTML and SVG elements, but MathML elements are their own type in JS: MathMLElement.

In general, we should support MathML tags and attributes in a similar manner that we support SVG tags and attrs.

Tasks:

• Create MathMLElement interface in scalajs-dom (Proposal for MathML base class scala-js/scala-js-dom#858)
• Update Scala DOM Types
  • Add MathML tags and attrs to SDT (WIP: Math ml tag defs scala-dom-types#105)
  • Add mathml example to SDT tests
  • Fix type of tag: It should be MathMLelement, not HTMLelement
  • Fix xmlns prop – I think it should be an attribute? But not just an HtmlAttr – it needs to work for all elements.
• Add MathML keys, types and helpers to Laminar (similarly to what we have for HTML and SVG)
  • I think all mathml tags should be hidden behind mathml namespace, similar to how svg is organized.
  • The same goes for attributes, I think
  • Add MathML example to Laminar tests