docs: change .eslintrc.js to eslint.config.js

[BrunoMollo]

Just a small update to the docs. Nowadays, npm create svelte@latest creates a project with a eslint.config.js, not a .eslintrc.cjs. This has been updated lately in eslint v9.0.0 and changed in the framework in this PR.

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[eltigerchino]

Thank you!