Expose the Either parser

[Geal]

This will make it easier to build Parser implementations without having to implement the trait manually

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.

Project coverage is 61.42%. Comparing base (a44b52e) to head (5534e57).

Files with missing lines	Patch %	Lines
benchmarks/benches/json.rs	0.00%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1821      +/-   ##
==========================================
+ Coverage   61.34%   61.42%   +0.07%     
==========================================
  Files          26       26              
  Lines        3128     3124       -4     
==========================================
  Hits         1919     1919              
+ Misses       1209     1205       -4     

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[codspeed-hq]

CodSpeed Performance Report

Merging #1821 will improve performances by 13.57%

_{Comparing either-parser (5534e57) with main (a44b52e)}

Summary

⚡ 1 improvements
✅ 23 untouched benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	number	244.2 ns	215 ns	+13.57%