Add stylelint to tooling setup

[troychaplin]

Description

Adding styleline as per a suggestion from @MaggieCabrera — #33 (comment)

Testing Instructions

1. Update dependencies: npm ci
2. Test style lint: npm run lint:css

Use of AI Tools

AI assistance: Yes
Tool(s): Claude
Model(s): Opus 5
Used for: determining the cause and appropriate fix for this error when adding stylelint and updating dependencies:

npm warn allow-scripts Run npm approve-scripts --allow-scripts-pending to review, or npm approve-scripts <pkg> to allow.

[github-actions]

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Co-authored-by: troychaplin <areziaal@git.wordpress.org>
Co-authored-by: MaggieCabrera <onemaggie@git.wordpress.org>

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[MaggieCabrera]

Thanks for picking this up! A few things:

• Use stylelint directly instead of @wordpress/scripts. The theme has no build step, so wp-scripts adds a lot of packages (and the new allowScripts and .prettierignore entries) just to get the stylelint binary. TT5 uses stylelint "**/*.css" with stylelint and @wordpress/stylelint-config as devDependencies.
• Extend @wordpress/stylelint-config rather than /scss. Both stylesheets are plain CSS.
• Set rule-empty-line-before and at-rule-empty-line-before to null like Gutenberg does. That removes all the blank-line edits in style.css (one of them is a line of four spaces).
• Override selector-class-pattern with Gutenberg's pattern (^[a-z][a-z0-9]*(?:(?:__|--|-)[a-z0-9]+)*$) instead of the inline disables. Block classes are BEM, so this will come up constantly.
• Add *.css to lint-staged.config.mjs and a CSS step to lint.yml, plus a lint:css bullet in the README. The $schema note in the README looks unrelated to this PR.

[troychaplin]

Thanks for picking this up! A few things:

Thanks @MaggieCabrera, requested changes have been pushed.

[MaggieCabrera]

Thank you! let's bring it in