Add collaborations frontmatter field

[stevejpurves]

Which area is this feature request for?

Command Line Tools

Describe the feature you'd like to request

Collaboration are distinct from affiliations and may be represented differently in both PDF outputs and website outputs.

For example: myst-templates/physical_review_journals#13

Describe the solution you'd like

Extend the frontmatter structure to add a collaborations fields as a sibling to affiliations.

Describe alternatives you've considered

overloading the use of affiliations for collaborations, but this doesn't allow for distinctions between the two in templates that require them.

[stevejpurves]

already fixed in #343 !