Handle .bgz file extension for text-index

[cmdcolin]

Currently the code explicitly checks for .gz, but this helps allow .gz extensions. It could be more advanced and detect a gzip encoding regardless of file extension but this may help

Fixes #2718

[codecov]

Codecov Report

Merging #2726 (3119063) into main (b43ed81) will decrease coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main    #2726      +/-   ##
==========================================
- Coverage   60.75%   60.73%   -0.02%     
==========================================
  Files         561      561              
  Lines       26346    26346              
  Branches     6424     6424              
==========================================
- Hits        16007    16002       -5     
- Misses      10013    10018       +5     
  Partials      326      326              

Impacted Files	Coverage Δ
products/jbrowse-cli/src/types/gtfAdapter.ts	0.00% <0.00%> (ø)
products/jbrowse-cli/src/types/gff3Adapter.ts	97.67% <100.00%> (ø)
products/jbrowse-cli/src/types/vcfAdapter.ts	91.66% <100.00%> (ø)
...nts/src/SNPCoverageRenderer/SNPCoverageRenderer.ts	66.05% <0.00%> (-5.51%)	⬇️
packages/core/util/analytics.ts	91.83% <0.00%> (+2.04%)	⬆️

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