Emoji in headers should be excluded when generating ids

Summary

Emoji characters in headers should be excluded when generating ids

Steps to reproduce

In a GitLab wiki page, create a heading that contains emoji. For example, ## Terrible Bugs :scream_cat:.

What is the current bug behavior?

The heading will correctly look like: "Terrible Bugs 🙀", but the id will be terrible-bugs-scream_cat.

What is the expected correct behavior?

The heading should look like: "Terrible Bugs 🙀", and the id should be terrible-bugs.

This is a bug because, not only is this behavior undesirable, it is inconsistent with the documentation. The documentation for how GitLab Markdown creates headers states that "All non-word text (such as punctuation or HTML) is removed" when creating the ID. Emoji are "non-word text" just like punctuation, and so should be removed.