Add a test covering the highlighting of non-contiguous spans

[stof]

I ported this package to PHP as part of the scssphp project (as I'm porting dart-sass which uses this package). When collecting code coverage for the testsuite of my PHP port, I noticed that the code dealing with non-contiguous lines was not covered by the tests.
This PR adds a test covering this case.

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[stof]

Is anything missing here ? Can I get some feedback ?

[mosuem]

Closing as the dart-lang/source_span repository is merged into the dart-lang/tools monorepo. Please re-open this PR there!

[mosuem]

Also, I just saw that this PR never got a review. Hopefully the monorepo transfer will help in not having PRs like these fall through the cracks!

cc @devoncarew