fix: encode large negative numbers in stringify

[deepview-autofix]

Numbers with magnitude >= 1e21 are serialized by JavaScript in exponential notation (e.g. -1e+22). The previous check only compared value < 1e21, so negative numbers in this range skipped encodeString and left a raw '+' in the output, which query string parsers decode as a space and corrupt the round-trip. Use Math.abs to apply the threshold symmetrically for positive and negative values.

[ChALkeR]

Prior to this fix:

> require('fast-querystring').stringify({x:-1e100})
'x=-1e+100'
> require('querystring').stringify({x:-1e100})
'x=-1e%2B100'

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.97%. Comparing base (9dcbaf2) to head (104c05d).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #93   +/-   ##
=======================================
  Coverage   68.97%   68.97%           
=======================================
  Files           7        7           
  Lines         448      448           
  Branches       72       72           
=======================================
  Hits          309      309           
  Misses        136      136           
  Partials        3        3           

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