fix: prevent binding to imports

[Rich-Harris]

Alternative to #13033, fixes #13027 — prevents binding to imports, just like we already prevent binding to other constants (including derived values).

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[dummdidumm]

Makes sense (Svelte 4 errors, too), but template is not a valid value yet (at least from the types)?

[Rich-Harris]

yeah i put it in the generated file, d'oh. fixing