fix(jsx): allow a function component to return an array

The "Kept in sync" comment is honest — but having to say it is the smell. If someone adds a return variant to FC and forgets ElementType, nothing fails at compile time. A shared type alias (type FunctionComponentReturn = ...) consumed by both would make drift structurally impossible.

This PR changes or adds ElementType (src/jsx/base.ts) but I don't see a test for this definition in the diff, and the symbol index shows no existing test that already exercises it. Did you mean to add a test? Bound: this is a diff + index heuristic — if this definition is covered by a test this read couldn't see (an integration/e2e suite, a differently-named test, or a test in another PR), please dismiss this.

The JSX.ElementType export is the new public API surface here. Is there a plan to document it alongside FC in the JSX type docs, or is it considered an internal detail of TypeScript's JSX resolution that users shouldn't reference directly?