feat(yaml): allows omitting value in explicit mapping

[l0ngvh]

Summary

Allows omitting value in an explicit mapping entry

Test Plan

The parser can now parse the following example:

? a
? b

Which is equivalent to

{
  "a": null,
  "b": null
}

[codspeed-hq]

CodSpeed Performance Report

Merging #6054 will not alter performance

_{Comparing vohoanglong0107:yaml-explicit-mapping-empty-value (d88a01e) with main (a82a1f2)}

Summary

✅ 94 untouched benchmarks
🆕 1 new benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
🆕	html_formatter[index_1033418810622582172.html]	N/A	462.7 µs	N/A