web: seal CSS modules migration with linter rules

Created by: valerybugakov

Context

The CSS modules migration was completed at the beginning of the month, but we didn't have time to seal it with linter rules that would allow closing the issue. This PR addresses that.

👆10 Global CSS files left are not meant for migration because they introduce global styles not tied to any specific React component.

Changes

• Added new package stylelint-plugin-stylelint to the client folder.
• Added one custom stylelint rule filenames/match-regex.
• Enabled this rule to ban non-module SCSS files outside of designated folders.

Notes

• This PR should be merged after updating our stylelint-config: https://github.com/sourcegraph/stylelint-config/pull/153
• More context about the migration can be found in the CSS isolation RFC.
• The Code Insight lives on the Frontend Platform migrations dashboard.

Closes the tracking issue https://github.com/sourcegraph/sourcegraph/issues/23010. 🤩