chore: cleanup some old unused code (#7856)

* chore: remove unused `@stylelint/postcss-css-in-js` dependency

* chore: remove unused `all-contributors-cli` dependency

* refactor: use built-in URL constructor instead of `url` dependency

* chore: remove unused e2e test file

* chore: remove unused build dependencies

* chore: remove unused files

* chore: remove unused MarkdownControl code

* chore: delete more unused Markdown widget code

* chore: delete unused richtext widget code

* chore: remove unused `browserify` dependency

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Co-authored-by: Yan <61414485+yanthomasdev@users.noreply.github.com>

Author: delucis