DITAVAL support for @outputclass via proprietary @style token

[jelovirt]

DITA 1.3:

The following tokens SHOULD be processed by all DITAVAL processors:
…
In addition, processors might support other proprietary tokens for different types of styling.

What if we supported prefixing the value with a period (.) as a proprietary token and we’d treat that as if DITAVAL had an @outputclass attribute? So instead of

<revprop action="flag" color="red" val="review-a">

You could use

<revprop action="flag" style=".review" val="review-a">

And your output CSS would have

.review {
  color: red;
}

DITA 2.0 adds a feature to use @outputclass, so this would only be needed for DITA 1.x. This would make it possible for people to start using CSS classes with DITA 1.x and then migrate to @outputclass when they switch to DITA 2.0.

Related to #3482

[jelovirt]

We've already supported @outputclass since 3.5 and because DITAVAL files rarely use a DTD, maybe we can just direct DITA 1.x users to DITA 2.0 DITAVAL.