feat: add forget method to settings extender

[OrdinaryJellyfish]

Fixes #3934

Changes proposed in this pull request:

• Adds a forget method to settings extender. This method allows a callback returning a bool, which if returns true, deletes the setting so the default is used.

Reviewers should focus on:

• Whether this new extender was written properly
• If the forget method properly deletes a setting when callback returns true

Screenshot

QA

Necessity

• [ ] Has the problem that is being solved here been clearly explained?
• [ ] If applicable, have various options for solving this problem been considered?
• [ ] For core PRs, does this need to be in core, or could it be in an extension?
• [ ] Are we willing to maintain this for years / potentially forever?

Confirmed

• [ ] Frontend changes: tested on a local Flarum installation.
• [x] Backend changes: tests are green (run composer test).
• [ ] Core developer confirmed locally this works as intended.
• [x] Tests have been added, or are not appropriate here.

Required changes:

• [ ] Related documentation PR: (Remove if irrelevant)
• [ ] Related core extension PRs: (Remove if irrelevant)

[OrdinaryJellyfish]

Sounds good to me!