[2.x] chore: add xslt polyfill

[imorland]

Changes proposed in this pull request:
All browsers have deprecated XSLT, and will start to remove it soon. s9e/textformatter uses this, but at the moment there is no official solution. See s9e/TextFormatter#250

https://developer.chrome.com/docs/web-platform/deprecating-xslt

Reviewers should focus on:
This PR adds a polyfill to restore the XSLT functionality and is intended as a short term solution until s9e push an update

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)