[EuiCodeEditor] Enable exiting with esc for all modes

[thompsongl]

Summary

Fixes #3819, which highlighted an un-exit-able EuiCodeEditor when enableLiveAutocompletion=true.

The problem stemmed from the this.aceEditor.editor.completer logic, which will always be present when enableLiveAutocompletion=true regardless of the actual presence of an autocomplete element. There is no other analog to discern a scenario where esc is hit and an autocomplete is open.

The solution is the use the react-ace commands interface, which does not execute when an autocomplete is open. The previous onAceKeydown logic remains because commands also do no execute in read-only mode.

Checklist

- [ ] Check against all themes for compatibility in both light and dark modes
- [ ] Checked in mobile
- [ ] Checked in IE11 and Firefox
- [ ] Props have proper autodocs
- [ ] Added documentation

• Checked Code Sandbox works for the any docs 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

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3833/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3833/

[myasonik]

🚀