Add Stylelint to lint CSS

[connor-baer]

Purpose

SumUp is slowly replacing Emotion.js with CSS Modules. One benefit of writing vanilla CSS in separate files is that we can use Stylelint to catch errors and enforce a consistent code style.

Approach and changes

• Install Stylelint and some plugins
• Create a sensible base config

Definition of done

• Development completed
• Reviewers assigned
• Unit and integration tests

[changeset-bot]

🦋 Changeset detected

Latest commit: 2cf9d8c

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

This PR includes changesets to release 1 package

Name	Type
@sumup/foundry	Minor

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[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (ebbb437) 68.01% compared to head (2cf9d8c) 68.91%.

Files	Patch %	Lines
src/configs/stylelint/config.ts	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #916      +/-   ##
==========================================
+ Coverage   68.01%   68.91%   +0.90%     
==========================================
  Files          18       19       +1     
  Lines         372      386      +14     
  Branches       63       67       +4     
==========================================
+ Hits          253      266      +13     
- Misses        110      111       +1     
  Partials        9        9              

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