the four regex rules could only reach a fixed nesting depth so an object
inside a condition or a brace inside a string still got through. this
walks the value from the prop until whatever ends its own syntax so
there is no depth limit any more.
it also drops two over guards the regexes had. a class inside a call in a
variant prop migrates again and so does a class next to an attribute that
only mentions variant in its text.
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