[fix] collapse whitespace in class / style attribute

[tanhauhau]

collapse whitespace within class and style attribute of an element

Fixes #6004

Before submitting the PR, please make sure you do the following

• Prefix your PR title with [feat], [fix], [chore], or [docs].
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with npm test and lint the project with npm run lint

[Conduitry]

As I mentioned on the original issue #6004, I still have concerns about forcing this normalization on people, and it sounds like @dummdidumm had concerns too, and the Prettier plugin stopped doing this reformatting because of that.

Maybe we could have this controlled by the same collapseWhitespace option we already have, but that still feels a bit dangerous.

[tanhauhau]

But the collapsing only done for class and style attribute for elements, which shouldn't matter right?

[Conduitry]

I don't know. I assume dummdidumm had some reason for stopping reformatting class attributes in the Prettier plugin.

[dummdidumm]

The world has changed, and so has my stance on this. I think collapsing whitespace inside style/class attributes is fine, since according to the spec it doesn't make a difference. The latest version of prettier now even tries to group class attributes by prefixes into multiple lines if it doesn't fit into one line ( prettier/prettier#7865 ; whether to add that to prettier-plugin-svelte is another thing, seems like quite a few people are not happy with it ).