[meta] Exclude `fs` from browser bundlers

[fregante]

As suggested in https://github.com/substack/tape/issues/561#issuecomment-909469015 and verified in https://github.com/substack/tape/issues/561#issuecomment-917700972

In short this change will let:

• webpack 4 users bundle without any configuration
• webpack 5 users remove 1 line of config 😅
• other bundlers to skip looking for the fs dependency altogether

[codecov-commenter]

Codecov Report

Merging #565 (00f21ea) into master (c837b02) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #565   +/-   ##
=======================================
  Coverage   96.49%   96.49%           
=======================================
  Files           4        4           
  Lines         627      627           
  Branches      145      145           
=======================================
  Hits          605      605           
  Misses         22       22           

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[fregante]

Thanks!