feat: create reusable Hero component

[DavideIadeluca]

Fixes #4202

Changes proposed in this pull request:

• Create new reusable hero component to streamline the implementation of each variation

Reviewers should focus on:

• Tried to keep the core abstract implementation as lightweight as possible
• Perhaps some methods or the new class itself could be named better
• Does this change need to be documented?
• Are there any other Hero implementations i've missed?

Screenshot
DOM / UI should stay the same

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)

[DavideIadeluca]

FYI @imorland I just made a minor change I've had in mind. This PR is now good to go from my side