Don't always stringify attrs

[Crisfole]

Is your feature request related to a problem? Please describe.

It's irritating that html elements being rendered with attrs get stringified when the value being passed them is falsey. Specifically in the null | undefined | false case. Currently null and undefined act as expected (they just skip the attr), but false gets serialized as "false" which is actually truthy.

<a {href} {download}>Download</a>

Describe the solution you'd like

Treat false like undefined and null are currently. Warn when a non-string value is being automatically stringified for an html element. You almost never want [Object object] in your html. You almost never want the elements of your array indiscriminately joined with commas in your html.

Describe alternatives you've considered
I can easily fix this by using {download || undefined} but I'd really rather not.

How important is this feature to you?
I don't think it's important to me personally because the fix is easy. I do think it's a gotcha that could affect adoption.

Additional context

Link.svelte

<script>
  export href;
  export let download = false;
</script>

<a {href} {download}>Download</a>

Consumer.svelte

<script>
  import Link from './Link.svelte'
</script>

<Link href='www.google.com'>Google</Link>
<Link href='https://www.youtube.com/watch?v=dQw4w9WgXcQ' download>Video</Link>

Above, the google link gets a download=false attr, and the video gets a download=true. In both cases they're considered download links. Even worse, passing download={false} still fails the same way.

[Conduitry]

As described in this comment and the one after it, treating false as a missing/skipped attribute would probably be a breaking change, and would also prevent the usage Josh mentions in the second comment.

What I recall from going through the known attributes for #3013 is that download is not a boolean attribute, and so doesn't (and shouldn't) have the special handling we have for some other attributes like disabled or selected.

I really don't think we want to change the behavior here, but it should be mentioned somewhere in the docs.

[Conduitry]

Adding the 'docs' label so we have something to remind us to mention which how to get certain attributes to not render.