Initialise html/text bindings from DOM

[Rich-Harris]

This is a branch off of #2379 (which apparently I merged already? oops, didn't mean to yet) which initialises html and text bindings from the DOM, if the bindings don't yet have values (just as we do with e.g. <select>). I also tided up some other SSR attribute stuff while I was in the neighbourhood

[Conduitry]

We should also mention these bindings here and maybe also in the tutorial.

[Rich-Harris]

While adding docs it occurred to me that there's no real reason text and html should be special cases — with almost all other bindings (except bind:group and bind:this) the binding name corresponds to the property (input.checked, video.currentTime etc). So I renamed them to textContent and innerHTML

[normano]

In the future, may want to add a note that the bindings were renamed since version x as these bindings were not working for me and I questioned if the doc was correct.