Add Learn CI/CD syntax template picker

What does this MR do?

This MR adds a Learn CI/CD syntax picker when editing the .gitlab-ci.yml file. This will be run as an A/B experiment to see whether or not it drives creation of pipelines.

This can be tested locally by visiting a local project and appending the following to the URL:

/-/new/master?commit_message=Add+.gitlab-ci.yml&file_name=.gitlab-ci.yml&force_experiment=ci_syntax_templates

For example:

http://localhost:3000/user/project/-/new/master?commit_message=Add+.gitlab-ci.yml&file_name=.gitlab-ci.yml&force_experiment=ci_syntax_templates

Screenshot

Note: There is no changelog, since this will only be run as an experiment (not available for all users) on dotcom for now.

Issue: #281057 (closed)