Initialize EuiCodeEditor with existing mode

[thompsongl]

Summary

Work in #2577 happened upon a 404 error related custom modes in EuiCodeEditor (and prevents its docs from building statically). When setting a custom mode, EuiCodeEditor would initialize without a mode and use setMode on componentDidMount, resulting in an attempt to fetch [some_path]/mode-mode.js, which does not exist (error visible in prod docs, also).

The react-ace docs specify that the component should be initialized with an existing mode prior to setting the custom mode. This prevents the fetch of a nonexistent mode file.

Fixes #2524

Checklist

- [ ] Checked in dark mode
- [ ] Checked in mobile
- [ ] Checked in IE11 and Firefox
- [ ] Props have proper autodocs
- [ ] Added documentation examples
- [ ] Added or updated jest tests

• Checked for breaking changes and labeled appropriately

- [ ] Checked for accessibility including keyboard-only and screenreader modes

• A changelog entry exists and is marked appropriately

[thompsongl]

Just a note that this will also close #2524

[chandlerprall]

Changes LGTM; Pulled and tested locally. One small ask.

Nice, clean fix for an interesting ACE issue!