feat: add custom element with support for animated WebP images

[mercihabam]

Thanks for making a pull request to converse.js!

Before submitting your request, please make sure the following conditions are met:

• Add a changelog entry for your change in CHANGES.md
• When adding a configuration variable, please make sure to
  document it in docs/source/configuration.rst
• Please add a test for your change. Tests can be run in the commandline
  with make check or you can run them in the browser by running make serve
  and then opening http://localhost:8000/tests.html.

Related: #3909

[jcbrand]

Thanks @mercihabam, looks like you're not finished yet?

[mercihabam]

Yes, not yet done. There are still tests to add, and then get the PR out of draft.

[mercihabam]

But it's one of the best times to give insights if any 🫠

[mercihabam]

@jcbrand this is now ready. Thanks!