Interactive editing of `.gitlab-ci.yml`

Game plan

• mini-snippets (snippets that don't completely replace the entire file's content, but just add a small portion.. like a stage or a review app etc to the file you were already editing) - #26337 (closed)
• auto-completion (even if difficult, i think this would single handedly improve a lot.. I just love this feature within framer for example) - #26336 (moved)
• continuous live CI linting (the CI linter is nice, but this should be continuous and live updating, immediately spotting smaller errors and mistypings as far as GitLab can detect them) - #25676 (moved)

Links

• From https://gitlab.com/gitlab-org/gitlab-ce/issues/20785