rewrite/refactor bigwig and bigbed stores as node modules

[cmdcolin]

This is a combo-issue to refactor BigWig and BigBed stores as node modules

They include similar code and data structures actually so they're a bit intertwined, it is some question of whether bigbed and bigwig are separate files or the same module

[rbuels]

should do #1143 at the same time as this, with the newly thin JBrowse store applying the subfeature transformations