Search: enable for ko; fix ja; hide for zh-*

[microbit-robert]

• Omit lunr warnings emitted when changing languages.
• Hide search where it is not supported for a language (currently zn, ko).

Closes #949.

[github-actions[bot]]

Preview build will be at https://review-python-editor-next.microbit.org/lunr-fixes/

[microbit-robert]

In draft as I will also look at the tokenizer issue for search input with Japanese as the selected language.

[microbit-matt-hillsdon]

I think I'm happy with this.

[microbit-matt-hillsdon]

With @microbit-robert for further testing after today's changes.

[microbit-robert]

@microbit-matt-hillsdon All looks good to me!