fix: dynamic namespace fixes and enhancements

[dummdidumm]

In #10006 we added more elaborate mechanisms to determine which namespace a given element is in. For <svelte:element> we added a "can't know at compile time" case and introduced a limited heuristic into the runtime.
This doesn't work for a few reasons:

• we're checking the parent's namespace to determine the current namespace, but the element itself could be the one that changes the namespace
• as mentioned in the previous comment already, on the first render we can't do any parent analysis
• it does not take into account the static component namespace

The last point is the crucial one: In Svelte 4, we're falling back to the component namespace if we can't know statically - e.g. if someone added <svelte:options namespace="svg"> then <svelte:element> should fall back to that namespace instead.
We were not doing that up until now, which introduced a regression. Fixing this also means getting rid of the (flawed) "can't know statically" heuristic.

Fixes #10858, though for a complete solution we need some way to tell <svelte:element> the namespace at runtime through a special attribute. I chose the xmlns attribute for that like we do in the static case. So if you set that <svelte:element> will use that to get the namespace.

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