adding support for nonstandard map key in the decoder

[PejmanNik]

Adding support for nonstandard map key in the decoder by adding a new callback function in the decoder options that accepts the decoded map key and returns a JS standard map key (string or number)

fix#255

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.28%. Comparing base (1f9e335) to head (59057ee).
Report is 54 commits behind head on main.

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Additional details and impacted files

@@ Coverage Diff @@## main #266 +/- ## ========================================== - Coverage 98.00% 97.28% -0.73%  ========================================== Files 16 17 +1 Lines 1102 1140 +38 Branches 251 253 +2 ========================================== + Hits 1080 1109 +29 - Misses 22 31 +9 

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[gfx]

Thank you for your contribution. I like it. LGTM.

[gfx]

Released v3.1.0 with this PR.