Rename viewer -> pager

[LucasPickering]

Description

Describe the change. If there is an associated issue, please include the issue link (e.g. "Closes #xxx"). For UI changes, please also include screenshots.

When I originally implemented the pager functionality in #414, I didn't realize "pager" is the common term for this. I had seen the term before but thought "viewer" was more intuitive. I have since realized I was wrong. I've renamed the field in the config, code and documentation. Additionally, I realized PAGER is the common env var for this, so I added support for pulling from that, in the same way we pull from EDITOR.

Known Risks

What issues could potentially go wrong with this change? Is it a breaking change? What have you done to mitigate any potential risks?

This is very low risk because the old field name is still supported. Nothing will change about any existing user's workflows.

QA

How did you test this?

Tested manually

Checklist

• Have you read CONTRIBUTING.md already?
• Did you update CHANGELOG.md?
  • Only user-facing changes belong in the changelog. Internal changes such as refactors should only be included if they'll impact users, e.g. via performance improvement.
• Did you remove all TODOs?
  • If there are unresolved issues, please open a follow-on issue and link to it in a comment so future work can be tracked