feat: prevent unparenthesized sequence expressions in attributes

[Rich-Harris]

We'd like to reserve the possibility of doing something with sequence expressions in attributes in future, and this requires us to treat them as disallowed in the meantime. They still work but they need to be wrapped in parens.

Question: should this apply universally, or just in runes mode?

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Latest commit: e9567e8

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[dummdidumm]

Just in runes mode because who knows who depends on that in old code

[brunnerh]

Getting an error on this 🤔

<svelte:options runes />
<input value={(1, 2)} />

REPL

Works for components:

<svelte:options runes />
<script>
	import Child from './Child.svelte';
</script>
<Child value={(1, 2)} />
<Child value={1, 2} />

REPL