Rename legacy.componentApi to compatibility.classComponent

[Rich-Harris]

Describe the problem

We replaced the Svelte 4 legacy option (which added IE compatibility at the cost of bloatier output) with a completely different legacy option. Per #12100, this is a bit confusing.

Describe the proposed solution

Rename legacy.componentApi to compatibility.classComponent (but keep classComponent deprecated).

I'm not 100% sure this is the right name, since it also affects SSR output (which was never a class, just an object), so am open to alternatives.

Importance

nice to have

[abdelfattahradwan]

How about compatibility.legacyComponent? This name seems to convey better that the feature provides legacy compatibility without implying that it's tied to a specific implementation (like class components). It also maintains consistency by keeping the "legacy" part of the name, which might be less confusing for users familiar with the current terminology.