Cm6 themes v2

[khanniie]

Migrates editor theming CSS for Codemirror v6.

Updates class names to use the v6 classnames and adds a highlight plugin to codemirror so that it gives syntax class names that we can assign our own themed colors to.

I moved a lot of the themed code (_p5-contrast-codemirror-theme.scss, _p5-light-codemirror-theme.scss, _p5-dark-codemirror-theme.scss) into themify (variables.scss) for consistency.

Changes:

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• has no typecheck errors (npm run typecheck)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123
• meets the standards outlined in the accessibility guidelines

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• p5.js-web-editor
  • app-ted121a.release.editor.p5js.org/
  • preview-ted121a.release.editor.p5js.org/

🔧Environment Status : https://app.release.com/public/Processing%20Foundation/env-bef0afdae1