feat: allow use of any tag in listItems helper

[davwheat]

Changes proposed in this pull request:

This PR allows the use of any tag with the listItems helper function. This is useful to easily map an ItemList into separate DOM nodes outside of a <ul> element, without the need of code duplication.

Not everything needs to be a list. In fact, using <ul> and <li> too much is a common issue across the web, and is the reason why WebKit (Safari on macOS and iOS) actually remove all semantics from list tags when their list-style is set to none. (Source)

This change could help to prevent such "list-itis" within Flarum and its extensions.

Reviewers should focus on:

Screenshot

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)