feat: auto-open a tbody when we detect we need it

[dummdidumm]

WIP because it currently fails because the dom walking mechanism needs to know that it should walk differently in that case

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[FoHoOV]

IMHO its better when I know I did something wrong to go and fix it, rather than it being automatically fixed and me not knowing it. If this is the chosen direction of things, can we have warning when svelte repairs markup?

1. current behavior when I immediatly know I did something wrong.
2. this PRs behavior when everything just works and we don't get a warning or anything

IMHO this is the lands of unkown magic where harry potter lives :D

[Rich-Harris]

Having thought through this a bit more I've changed my tune — there's too much scope for ambiguity. Like, if I have something like this...

<table>
  <tr>...</tr>
  {#each things as thing}
    <tr>...</tr>
  {/each}
  <tr>...</tr>
</table>

...where do the <tbody> and </tbody> go? So I too come down on the side of forcing the developer to be explicit. I might go so far as to say that cases that are currently just warnings, like a <tr> inside an {#each ...} inside a <table>, should instead be errors

[dummdidumm]

They are only warnings because it won't cause DOM repairing when you're not using SSR. In case you do, it will fail with an explicit error on SSR at runtime.