fix: insert comment before text in an each block, to prevent glued nodes

[Rich-Harris]

fixes #12913

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[trueadm]

Can't we only do this if preserveWhitespace is true to avoid the extra anchors?

[dummdidumm]

That would break the case where you have only text nodes:

{#each 'abc' as l}
	{l}
{/each}

It doesn't result in a hydration mismatch, likely because it's repaired, but from debugging I can clearly see it wrongly fuses the text nodes together and hydration does destroy and recreated nodes.