Add option to ignore mismatched array lengths due to `null`/`undefined`.

[rodovich]

This change adds an option array.ignoreLength which, when set to true, prevents adding n to tables decoded from arrays with null or undefined entries, such as "[null,42]".

This allows iterating over the tables using pairs without encountering a key/value pair that was not part of the JSON data.

The option defaults to false to preserve the current behavior for any existing callers.

[codecov-io]

Codecov Report

Merging #42 into master will increase coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
+ Coverage   95.98%   95.98%   +<.01%     
==========================================
  Files          18       18              
  Lines        1021     1022       +1     
==========================================
+ Hits          980      981       +1     
  Misses         41       41

Impacted Files	Coverage Δ
lua/json/decode/composite.lua	100% <100%> (ø)	:arrow_up:
lua/json/decode/state.lua	100% <100%> (ø)	:arrow_up:

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