Frontend: Add predefined variables to the linter schema

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Summary

Users expect predefined variables to show up on the pipeline editor's autosuggest list.

The ones that come up in this video is because the variables have been used before somewhere in the file (for example, $CI_REGISTRY_IMAGE is already used in the IMAGE_TAG). It looks like the editor automatically adds user-defined variables in its autosuggest list. However, it won't suggested predefined variables that are not yet used in the file, such as $CI_REGISTRY

For context, this issue was brought up in a slack conversation a while back.

Proposal

We should add the predefined variables to the CI schema. We have the variables documented here