allow to define the prefix for generated names

[jantimon]

I've added support for configuring the prefix used by next-yak for generated CSS classes and variables
This can help in identifying yak-generated classes in tests or to avoid collisions with other libraries

The default prefix remains y in production and empty in dev mode, so this is fully backwards compatible

If needed you can now configure a custom prefix:

// next.config.js
export default withYak({
  prefix: 'my-app-'
});

The patch also adjusts the dev mode variable prefix from 'yak_' to 'var_' which makes a bit more sense semantically

[changeset-bot]

🦋 Changeset detected

Latest commit: 50b1eb5

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
yak-swc	Minor
next-yak	Patch

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Click here if you're a maintainer who wants to add another changeset to this PR

[codspeed-hq]

CodSpeed Performance Report

Merging #225 will degrade performances by 12.16%

_{Comparing feature/custom-prefix (50b1eb5) with main (eabe5d8)}

Summary

❌ 1 (👁 1) regressions
✅ 1 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	feature/custom-prefix	Change
👁	render KanjiLetterComponentYak	280.7 ms	319.5 ms	-12.16%