Use terminal editor for Edit Collection action

[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.

Instead of using the default system editor for .yml files when launching the Edit Collection action, we now defer to the $VISUAL/$EDITOR env vars. This means we will probably open a text-based editor like vim, where previously it was likely a GUI editor like VSCode. This could potentially be annoying but I think in most cases will be more convenient for users. If we get complaints we can make it configurable.

This also includes a new utility EnvGuard, which should be used in tests that access environment variables. Since the process environment is global mutable state, tests that access and modify it can fight with each other. This mutex ensures the tests each take their turn. It also makes it easy for them to clean up after themselves.

Closes #262

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 a change in functionality that could potentially break someone's workflow. In general I think it will be an improvement but it could be annoying for some users.

QA

How did you test this?

Manual testing in TUI, added some unit tests

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