Add ARXIV variable

[jheuel]

Description

This adds an identifier for arXiv "ARXIV" similar to PM(C)ID, DOI or ISBN.

See typst/hayagriva#326.

Type of change

• Variable addition (adds a new variable or type string)

Checklist

• I have installed the repo pre-commit hook; if not, and I have modified any of the schema files, I have run trang and/or prettier on the files, per CONTRIBUTING
• I have performed a self-review of my own code
• I have included suggested corresponding changes to the documentation (if relevant)