Create concept of "expanding" certain types of discontinuous displayed region boundaries

[cmdcolin]

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If we implement the collapsed intron feature, then the collapsed areas are represented by a sequence of discontinuous displayedRegions

Sometimes the features are a little hard to see at the boundaries of those displayedRegions

We could implement a feature, kinda of similar to gitub "diff" views, where we could expand or "show more" around the displayedRegion boundaries

here is a screenshot of github for analogy

the blue arrows represent the action to "show more" in the up or down direction, and there is also "expand" to show the inbetween areas. example link https://github.com/GMOD/jbrowse-components/pull/4521/files