Support `.ownerMathElement` (or something akin to it)

Hopefully this is the correct place for such a request, let me know if not!

I'm looking to add support for rendering MathML elements w/ the correct namespace in Preact, but an issue I'm running up against is that it's quite challenging for us to figure out the namespace an element currently lives in. For a (rough & a bit silly) example:

<math>
    {condition ? <mi>1</mi> : <mo>2</mo>}
</math>

During the first rendering pass, we're able to keep track of the namespace as we delve into the children of <math>, however, when condition changes at some point in the future, we need to figure out the namespace before creating whichever element we're swapping to. This poses a bit of a challenge with the existing API.

With SVGs, we use .ownerSVGElement !== undefined which is pretty effective. Unfortunately, modern MathML doesn't seem to have an equivalent we could use, though it looks like MathML 2 supported this.

Is this by chance something worth supporting? Not quite sure yet how we can work around this as we have quite tight perf & size limitations.

Edit: We can use .namespaceURI to determine the existing namespace, but this is a bit less than ideal.

The issue points to Preact's src/render.js and its ownerSVGElement check; start there and compare it with the namespaceURI behavior described in the issue. A completed change would require a defined MathML namespace-detection API or equivalent and agreement on scope, but the request names no target file or test.