Add the golden ratio φ

[brianrodri]

Description of the change

Adds the golden ratio phi as a new mathematical constant.

• Inspired by Support constant for phi sharkdp/insect#246
• PR written using Add tau. #17 as a template

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Checklist:

• Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)")
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation
• Added a test for the contribution (if applicable)