Fix broken published build of jbrowse/development-tools

[garrettjstevens]

Adds a prepack script so the build gets properly created on publish.

[codecov]

Codecov Report

Merging #2600 (8f4fe32) into main (94fdfbc) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2600      +/-   ##
==========================================
- Coverage   60.81%   60.80%   -0.01%     
==========================================
  Files         558      558              
  Lines       25723    25723              
  Branches     6069     6069              
==========================================
- Hits        15643    15642       -1     
- Misses       9753     9754       +1     
  Partials      327      327              

Impacted Files	Coverage Δ
products/jbrowse-web/src/util.ts	27.27% <0.00%> (-3.04%)	⬇️

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