Deprecate unused evented utility

[fredden]

This is related to #2547, but is only step one of two listed there

Changes proposed in this pull request:
I found #2547 while looking for a task to get started with. This is my first contribution to this project.
The issue mentions marking evented as deprecated and removing this. This pull request marks that utility as deprecated so we can safely remove the same in the future. This pull request introduces no functional changes.

Reviewers should focus on:
I'm unsure what to suggest here as the changes shouldn't have any impact on existing functionality, but is instead future-planning.

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) - This seems like a sensible idea; I didn't find any references to evented in https://github.com/flarum/docs/search?q=evented

[askvortsov1]

Excellent, thank you very much! Aside from the suggestion in the code, the only minor idea I have is changing the wording to "the evented util is deprecated and will be removed in v2.0 of Flarum". Otherwise, LGTM!

As to other steps, no bundled extensions currently use this util, so no action is needed from the Flarum side, so this issue is effectively done with this PR. Thank you very much for the contribution!

[davwheat]

I'm hesitant towards this, personally.

I think we should use the existing debug warning helper we have, instead, to prevent firing unnecessary console messages in production forums: https://github.com/flarum/core/blob/master/js/src/common/helpers/fireDebugWarning.ts

Furthermore, I'd prefer us to use a more descriptive message for this and to store it in a variable rather than repeating it.

Maybe...

"The `evented` util is deprecated and will be removed in Flarum 2.0. For more info, please see https://github.com/flarum/core/issues/2547"

I know very few or no extensions actually use this, but it sets a precedent for future deprecations. (And is very similar to the tootips message.)

To be clear, there's nothing inherently wrong with this PR, and a big thank you goes out to @fredden. First time PRs are nerve-wracking, and I know that from experience! ❤️

[fredden]

Thanks for the feedback. I'll make the suggested changes and ask for another review.