chore: reduce redundant lexer methods

[arendjr]

Summary

I noticed a few methods in the JS lexer that are also present in the generic lexer, so I removed them in favor of the generic one.

Also, there was a tiny duplication now in the current_char_unchecked() method between the JS and Grit lexers, so I moved it to the generic one.

Test Plan

Everything should stay green.

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[codspeed-hq]

CodSpeed Performance Report

Merging #2165 will degrade performances by 9.99%

_{Comparing arendjr:redundant-lexer-functions (99377dc) with main (a7ef325)}

Summary

❌ 1 regressions
✅ 92 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	arendjr:redundant-lexer-functions	Change
❌	big5-added.json[cached]	2.1 ms	2.4 ms	-9.99%

[ematipico]

Thank you! Nice clean up