feat: ItemList component

[SychO9]

Changes proposed in this pull request:

• Introduces an ItemList component to simplify the way we define itemlists while maintaining a more readable structure.
• If we decide to replace the component item lists in he code-base with this, it would be a major breaking change.
• We do have an auto upgrade tool planned so it won't be as bad if we maintain the same references to the item lists, namely Component.methodName as item list key.

Reviewers should focus on:

• Would we be alright with auto injecting context={this} using the webpack configuration?

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.

[SychO9]

@clarkwinkelmann would love to get your opinion on this one if and when you get the chance 🙏🏼

[luceos]

Left one remark. Seems good to go.

[luceos]

Are these changes necessary in this pr? Seems unrelated?