Warn if a recovering React user accidentally types 'className' instead of 'class'

[Rich-Harris]

Slightly tongue-in-cheek, but I've actually hit this myself occasionally while porting React components to Svelte:

[pngwn]

Opportunities for zing should never be passed up.

[Conduitry]

[pngwn]

That is a property not an attribute.

Edit: This was in response to a now deleted comment.

[finnbear]

I think this also applies to for instead of htmlFor and on:input instead of onChange

[Conduitry]

className and htmlFor will now both result in compiler warnings in 3.31.1.