Allow opening plaintext .vcf files from the "Add track" workflow

[cmdcolin]

This adds the VcfAdapter for a plain VCF to both CLI and add-track. It doesn't really try to warn against large VCFs but it should be fine for small VCF that are encountered in the wild. The VcfAdapter was created by @carolinebridge-oicr originally

We could think about also unifying the cli and core codebase, and making it more pluggable perhaps too...but maybe as follow ons :)

[codecov]

Codecov Report

Merging #2126 (5589107) into main (ec358f4) will decrease coverage by 0.02%.
The diff coverage is n/a.

❗ Current head 5589107 differs from pull request most recent head 98bb881. Consider uploading reports for the commit 98bb881 to get more accurate results

@@            Coverage Diff             @@
##             main    #2126      +/-   ##
==========================================
- Coverage   61.99%   61.96%   -0.03%     
==========================================
  Files         476      476              
  Lines       22781    22781              
  Branches     5351     5351              
==========================================
- Hits        14123    14117       -6     
- Misses       8377     8383       +6     
  Partials      281      281              

Impacted Files	Coverage Δ
packages/core/util/tracks.ts	64.86% <ø> (ø)
products/jbrowse-cli/src/commands/add-track.ts	53.27% <ø> (ø)
...nts/src/SNPCoverageRenderer/SNPCoverageRenderer.ts	80.24% <0.00%> (-7.41%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ec358f4...98bb881. Read the comment docs.