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.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

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

[SychO9]

Close enough but not quite ;)

The problem we are trying to solve is that, when the admin saves a value (empty string for example) that will be used as the setting value, even if an extension developer adds a setting that must be filled.

So the idea is that the callback will be used in the SetSettingsController class to determine which values to save and which to forget, because when an admin saves settings from the admin UI that's the controller that's used.

[SychO9]

Haven't tested yet, But this looks good!

Now all we have to do (as per the conversation with Sasha on the issue) is to inverse the intent (and the name) of the forget extender. So instead of saying forget this setting when the value equals this. It would become store this setting when the value is this.

For naming, maybe filter would make sense, what do you think?

[OrdinaryJellyfish]

Sounds good to me!