Allow multiple classes in the class-tuple syntax

[bicarlsen]

Is your feature request related to a problem? Please describe.
When using the class tuple syntax (i.e. class=("my-class", true)), multiple classes are not supported. e.g. class=("class-1 class-2", true).

Describe the solution you'd like
Extend the class-tuple syntax to allow for multiple classes.

Describe alternatives you've considered
I currently move each of the classes into its own tuple. e.g. class=("class-1", true) class=("class-2", true).

Additional context
The issue seems to come because DomTokenList.add_1(class_name) is used to add the class, which expects the classes to already be separated.

This could be remedied by first parsing the classes into a Vec, splitting on whitespace, then

1. Using DomTokenList.add() in place of add_1, or
2. By iterating over the Vec, repeatedly calling add_1
   I haven't played with the performance tradeoffs.

[bicarlsen]

If this seems like something worth tackling, I'm happy to issue a PR.

[gbj]

I am open to providing some alternate API to simultaneously set or un-set multiple classes in response to the same reactive change, and possibly checking for whitespace in the class name in the view macro and splitting them at runtime.

I'm not open to splitting the string at runtime for class:/class=(_, _) in general unless it's purely opt-in: if class:foo=some_condition requires checking whether "foo" contains whitespace or wrapping it in a JS Array, it's just pure overhead.

[bicarlsen]

I'll take a look and see what I can put together. Agreed that it should be done in the view macro.

[gbj]

Feature added by #2532. Nice work!