[css-highlight-api-1] Don't paint highlights that cross contain boundaries

[dandclark]

If a StaticRange crosses a contain boundary and the boundary point inside the contained element is removed, UAs would have to stop painting the full extent of the range. This would violate the contain invariant, since a change inside the element with contain would have affected the appearance of content outside of that element.

To eliminate this problem, never paint ranges that cross contain boundaries.

A slightly more scoped solution would be to apply this limitation only for highlights specified with StaticRange, but since there doesn't seem to be any use case for applying highlights across contain boundaries, the limitation is applied to live Ranges as well for consistency's sake.

This PR changes the range-painting criteria into an algorithm, since precisely expressing the concept of "crossing a contain boundary" is clumsy in prose.

Resolves #4598.

[ffiori]

Just a comment about style, when running Bikeshed to get a preview, it shows a fatal error:

FATAL ERROR: Line 514 isn't indented enough (needs 2 indents) to be valid Markdown:
"	   then return false."
 ✘ Did not generate, due to fatal errors

I think it can be fixed by using two tabs instead of tab+spaces.

[dandclark]

Just a comment about style, when running Bikeshed to get a preview, it shows a fatal errorm

@ffiori Thanks for pointing that out! Without noticing, I had been including a command line argument that was suppressing errors. Should be fixed now.

[sanketj]

@dandclark These changes LGTM, so I went ahead and approved. However, we should get consensus from the working group on this before merging.

[dandclark]

Abandoning this since we resolved in #4598 to ignore contain boundaries.