fix #3586 comments instead of empty text nodes (perf)

[neuronetio]

Comment nodes are much faster - no styles, drawing etc...

fix #3586

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Tests

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[benmccann]

I think this is a good idea. The vast majority of the comments on the linked issue support this change. The only possible hesitation was that text nodes are less cluttered when you open devtools - though I somewhat think the clutter is a good thin as it makes it clear what Svelte it doing and provides impetus for making it stop creating lots of empty text nodes. In the meantime, this is better performance

[dummdidumm]

I think using comment nodes would spam the browser elements view quite a bit. I feel like we should investigate using fewer of those empty text nodes instead.

[tivac]

I would love to see this change land as part of a dev vs prod style flagging system where the output in prod mode is allowed to be a bit uglier for the sake of speed/size/etc.