fix: pre tag with single \n incorrect hydration

[paoloricciuti]

Closes #14767

This is a super weird one...basically calling firstChild on a pre tag where the first text node is a single \n skips the \n completely. This meant that a code like this

<script>
	let name = $state('');
</script>

<pre>
<div><span>{name}</span></div>
</pre>

was incorrectly compiled to have

var pre = root();
var div = $.sibling($.child(pre));

but $.child was already returning the div.

This is not something we do, the browser does this as you can see in this codepen so we have to special case it. However this meant that the check for hydration mismatch on reset was wrong to so i had to compensate there too.

Finally for an extra bit of weirdness, the \n is literally removed from innerHTML too (also tested in the codepen) so in the test i had to specify an _expected.html.

All of this feels very weird and kinda brittle to me but i don't know if there's a better solution. WDYT?

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[Rich-Harris]

I think we can fix this more simply, and without any runtime impact: #14922

[paoloricciuti]

I thought about this but wanted to avoid messing with \n inside pre tags since they matter there...but it seems that it works anyway so maybe I was just a fool lol

[Rich-Harris]

Yeah this is just a quirk of HTML — a newline immediately following a <pre> is disregarded, it can have no effect:

document.body.innerHTML = `
<pre>
x
</pre>

<pre>x
</pre>
`;

const [one, two] = document.querySelectorAll('pre');

one.innerHTML === two.innerHTML; // true

[paoloricciuti]

Closing in favor of #14922