feat(api): subscription for genomic axis changes

[etowahadams]

Fix #910
Toward #

Change List

• Add a new API subscription location which returns the track ID and the genomic coordinates of a track when the track axis changes

Corresponding documentation PR

Example Spec

views: [
   { ..., tracks: [..., { type: 'brush', x: { linkingId: 'detail' } }]},
   { tracks: [{ ..., id: 'detail-track-1', x: { ..., linkingId: 'detail' } }]
]

Example API

gosRef.current.api.subscribe('location', (data) ⇒ {
  const { id, genomicPosition } = data;
  if(id === 'detail-track-1') {
      console.log('brush has been moved to ', genomicPosition);
   }
});

Checklist

• Ensure the PR works with all demos on the online editor
• Unit tests added or updated
• Examples added or updated
• Documentation updated (e.g., added API functions)
• Screenshots for visual changes (e.g., new encoding support or UI change on Editor)

[etowahadams]

This is where I've been testing the new location API. I noticed that there was commented out code for the other subscriptions so I did the same.

[etowahadams]

This is the extra option I added to this function so that it doesn't return "unknown" chromosome if the axis is out of the assembly bounds.

[sehilyi]

LGTM! Let's wait for the @ThHarbig's review

[sehilyi]

@etowahadams Let's merge this and make further patches depending on @ThHarbig's feedback in future

[etowahadams]

Sounds good! Will merge now