[CodeMirror v6] position fold gutter absolute + fix

[skyash-dev]

Issue:

Fixes #4165

Demo:

Changes:

• absolutely position the fold gutter and line number to prevent movement when line numbers are toggled
• reduce fold marker opacity

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