refactor(test): better infra for formatting

[ematipico]

Summary

This PR refactors the testing infrastructure of the formatter.

The infrastructure now uses the actual biome.json configuration, so we don't need to create an additional type to map the options. It was very inconvenient.

On the other hand, we can't group cases anymore, so I had to breakdown those tests by creating new folders, copying the files and setting the correct configuration.

Plus, I added a new CONTRIBUTING.md file that explains how to set up the infrastructure of a new formatter.

Test Plan

CI should pass.

Proofread the guide. https://github.com/biomejs/biome/blob/chore/better-formatter-testing/crates/biome_formatter/CONTRIBUTING.md

[netlify]

❌ Deploy Preview for biomejs failed.

Name	Link
🔨 Latest commit	68b24c3
🔍 Latest deploy log	https://app.netlify.com/sites/biomejs/deploys/6601b6740804960008b408c8

[codspeed-hq]

CodSpeed Performance Report

Merging #2189 will improve performances by 13.95%

_{Comparing chore/better-formatter-testing (68b24c3) with main (35ba975)}

Summary

⚡ 1 improvements
✅ 92 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	chore/better-formatter-testing	Change
⚡	eucjp.json[cached]	4.6 ms	4.1 ms	+13.95%