Enabled complex ci variable expression statements by default

See merge request gitlab-org/gitlab-ce!29904

(cherry picked from commit 1f3086a9)

f3b2dde8 Enabled complex ci variable expression statements by default

Refactored regex pattern matching to eagerly return tokens

Packaged behind a default-enabled feature flag and added operator documentation.

This makes ref validation to use always `UntrustedRegexp`.
This also splits the existing RubySyntax into separate
class.

Enables frozen string for the following:

* lib/gitlab/ci/*.rb
* lib/gitlab/ci/build/**/*.rb
* lib/gitlab/ci/config/**/*.rb
* lib/gitlab/ci/pipeline/**/*.rb
* lib/gitlab/ci/reports/**/*.rb

Partially addresses #47424.