feature: adds advanced link handling in core

[luceos]

Changes proposed in this pull request:

This PR adds rel and target to textformatter so that these can be easily extended and rendered into the source.

Without using the Extender the default values ngc nofollow are provided as a backward compatible way.

The new extender allows conditional overrides, a proof of concept extension is available at https://github.com/luceos/flarum-ext-dofollow; I will probably migrate this into the Blomstra namespace soon.

Reviewers should focus on:

• is the extender okay like this
• does the fallback work like before

Screenshot

Extension disabled

Extension enabled

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)