[New] parse: add allowSparse option for collapsing arrays with missing indices

[1cheese]

Fixes #181

[ljharb]

This will definitely need some tests.

[ljharb]

I've rebased this for you.

As for PR requirements, this would also need the readme to be updated.

However, #181 is marked "question" because I'm still not convinced that it's a good idea to create sparse arrays for any reason, and I still believe that all the use cases listed in that issue are better solved by altering the field names.

[codecov]

Codecov Report

Merging #312 (b04febd) into master (b522d2e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #312   +/-   ##
=======================================
  Coverage   99.85%   99.85%           
=======================================
  Files           8        8           
  Lines        1348     1363   +15     
  Branches      164      166    +2     
=======================================
+ Hits         1346     1361   +15     
  Misses          2        2           

Impacted Files	Coverage Δ
lib/parse.js	100.00% <100.00%> (ø)
test/parse.js	99.78% <100.00%> (+<0.01%)	⬆️
test/stringify.js	99.74% <100.00%> (+<0.01%)	⬆️

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[ljharb]

I've rebased this, and added stringify tests to match the parse tests that you added (thanks).

I continue to think producing sparse arrays is a bad idea, but since stringify accepts them, it seems useful to enable a round trip.