feat: use bracket matching instead of ssr:n comments

[Rich-Harris]

Per #10609, it's simpler and more efficient to do this...

<![>stuff<!]>

...than this:

<!ssr:0>stuff<!ssr:0>

It's smaller and less noisy, but it's also much clearer which comments represent the start vs the end of a block. It also makes SSR more efficient, because we don't need to create a bunch of incrementing anchors — we can just use string literals.

The {@html '<p>invalid</p>'} tests were failing with that change due to some quirk or other — this was fixed by clearing out container.textContent in render.js on hydration failure rather than removing nodes individually (which for whatever reason left some nodes behind). I would expect clearing the text content to be more efficient anyway; it's certainly simpler.

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[PatrickG]

This is somewhat related.
It is about a {#each}-block inside of <svelte:head> which renders a <title> with the help of <svelte:element>. So the SSRd <title> tag ends up with HTML comments in it.
And it seems like it would still be a problem with this changes.

[Rich-Harris]

We're going to need a separate issue for the <title> thing