Enable webpack code splitting

What does this MR do?

Starts work on #33391 (closed) by enabling and testing webpack code splitting (using the es2015 dynamic import() method)

• use babel as the eslint parser to prevent it from complaining about stage 2 ECMAScript syntax like import()
• test that async imports can correctly find and load code-split chunks when using a non-default root url (relative_url_root setting) - closes #32989 (closed)
• convert emoji picker into a relative url chunk (after refactor in !12432 (merged))

Are there points in the code the reviewer needs to double check?

• eslint should not choke when it encounters import()
• emoji picker and PDF file preview should still work when testing with a relative_url_root setting
  • this may be tested in staging

Why was this MR needed?

see #33391 (closed) for a full explanation

Screenshots (if relevant)

n/a

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• [ ] API support added
• Tests
  • [ ] Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

related to #33391 (closed)
closes #32989 (closed)