Add ability to get promoter sequence and intron sequence for genes from the feature details panel

[cmdcolin]

This adds some more options to the sequence calculations in feature details

It can get 500bp up/downstream of the gene, and the intron sequence, or a "collapsed intron" with 10bp of the intron sequence around the splice site

Things you'll note while observing this include that upstream and downstream are fetched in a separate request to the main, because I thought it was a little tricky to get the math right for fetching the sequence in one request with the possibility of hitting the start or end of chromosome boundaries, so I figured I would ensure the integrity of the fetch for the main gene body and then do the upstream/downstream separately

[codecov]

Codecov Report

Merging #1814 (62c444d) into master (e2594e3) will increase coverage by 0.26%.
The diff coverage is 60.12%.

❗ Current head 62c444d differs from pull request most recent head 6e447c3. Consider uploading reports for the commit 6e447c3 to get more accurate results

@@            Coverage Diff             @@
##           master    #1814      +/-   ##
==========================================
+ Coverage   58.85%   59.12%   +0.26%     
==========================================
  Files         459      460       +1     
  Lines       21389    21437      +48     
  Branches     5051     5082      +31     
==========================================
+ Hits        12589    12674      +85     
+ Misses       8489     8449      -40     
- Partials      311      314       +3     

Impacted Files	Coverage Δ
.../core/BaseFeatureWidget/SequenceFeatureDetails.tsx	57.14% <57.14%> (ø)
...kages/core/BaseFeatureWidget/BaseFeatureDetail.tsx	76.61% <100.00%> (+34.46%)	⬆️
packages/core/util/index.ts	81.52% <0.00%> (ø)

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[cmdcolin]

has a couple more tests for the behavior of the sequence translation into protein, etc.

also added react-intersection-observer to only fetch the sequence on demand, so that if a gene with 100 transcripts loaded into the feature details, it avoids overwhelming the feature details panel (only loads data on-demand)

this package has a fairly small bundle (1.6kb gzip) and intersectionobserver has good browser support