PIPE-4728 Add is default branch pipeline value

[liamclarkedev]

Description

A new pipeline value that represents whether the branch the pipeline was triggered on is the default, following the below criteria:

• true when it is the default branch.
• false when it isn’t the default branch.
• false when it is any other push (e.g. tag push).
• omitted when it is not a VCS-triggered pipeline.

Reasons

A link to a GitHub and/or JIRA issue (if applicable).
Otherwise, a brief sentence about why you made these changes.

https://circleci.atlassian.net/browse/PIPE-4728

Content Checklist

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[rosieyohannan]

🎉 thank you!