Editor: Add Korean language.

[shinjeongmin]

hello, i'm Korean user of three.js

I'm using the three.js editor, and I think it would be great if we could use the UI text in Korean for the Korean users.

So, I checked the editor code, added a Korean (kr) language option in the sidebar settings script, and translated the corresponding texts in the Strings script to Korean.

There are quite a few Korean users who use the three.js editor, and I hope accept my suggestion to make it more convenient for them.

Below screenshots are from testing the code modifications I made on local.

※ It seems I made a mistake by committing directly to the dev branch and submitting a pull request without creating a new branch. I'm resubmitting it.

Modify

I was looking into contributions related to Korean translations and found that there is an ISO standard for Korean. I have made the necessary corrections accordingly.
changed kr -> ko according ISO standard #20877 (comment)

[Mugen87]

The following line needs an update as well:

three.js/editor/js/Config.js

Line 7 in b331d8e

const suggestedLanguage = [ 'fr', 'ja', 'zh' ].includes( userLanguage ) ? userLanguage : 'en';

When kr is added, Korean will automatically be selected as the default when the browser's language setting is Korean.

[shinjeongmin]

thanks! I didn't realize that navigator.language was located there.
I tested suggested language to 'ko' and it was well.
When I started three.js editor, it shows text to korean to me!