Adding NextToken() method to decoder

[marcsantiago]

Decoder: Add the ability to determine the next token. This is useful within the context of mixed JSON data in the UnmarshalJSONObject implementation

fixes https://github.com/francoispqt/gojay/issues/119

[codecov-commenter]

Codecov Report

Merging #147 into master will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #147   +/-   ##
=======================================
  Coverage   99.96%   99.96%           
=======================================
  Files          38       39    +1     
  Lines        5323     5339   +16     
=======================================
+ Hits         5321     5337   +16     
  Misses          1        1           
  Partials        1        1           

Impacted Files	Coverage Δ
decode_token.go	100.00% <100.00%> (ø)

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