Add non-indexed and plaintext VCF Adapter to variants plugin

[carolinebridge]

solves this issue for jbrowse-plugin-gdc while providing the functionality to core

[codecov]

Codecov Report

Merging #2068 (768b119) into main (d18857c) will increase coverage by 0.03%.
The diff coverage is 87.27%.

@@            Coverage Diff             @@
##             main    #2068      +/-   ##
==========================================
+ Coverage   61.41%   61.44%   +0.03%     
==========================================
  Files         479      480       +1     
  Lines       22908    22963      +55     
  Branches     5249     5264      +15     
==========================================
+ Hits        14068    14110      +42     
- Misses       8565     8578      +13     
  Partials      275      275              

Impacted Files	Coverage Δ
packages/core/util/tracks.ts	64.86% <ø> (ø)
plugins/variants/src/index.ts	95.23% <66.66%> (-4.77%)	⬇️
plugins/variants/src/VcfAdapter/VcfAdapter.ts	88.46% <88.46%> (ø)
...nts/src/SNPCoverageRenderer/SNPCoverageRenderer.ts	80.24% <0.00%> (-7.41%)	⬇️

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[cmdcolin]

could this code work if it was a plain text vcf? could possible check if there is the magic number in the buffer https://en.wikipedia.org/wiki/Gzip e.g. 1f8b and only unzip buffer in that case

[cmdcolin]

Can you update the PR title to be a "sentence style" title? We try to make them sort of like this cause they appear directly in our changelog (e.g. https://jbrowse.org/jb2/blog/2021/05/24/v1.3.0-release#130-2021-05-24)