3826: Flatten nested highlight ranges during DFS traversal r=matklad a=ltentrup
Implements the flattening of nested highlights from #3447.
There is a caveat: I needed to add `Clone` to `HighlightedRange` to split highlight ranges ~and the nesting does not appear in the syntax highlighting test (it does appear in the accidental-quadratic test but there it is not checked against a ground-truth)~.
I have added a test case for the example mentioned in #3447.
Co-authored-by: Leander Tentrup <leander.tentrup@gmail.com>
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