feat(language): add Khmer (@Earpengai)

[Earpengai]

Description

This PR adds support for the Khmer (Cambodian) language to Monkeytype.
The implementation includes:

1. Added khmer.json with:

• Common Khmer words used in daily conversation
• Proper language metadata (BCP47: km-KH)

2. Updated required configuration files:

• Added khmer.json to languages folder
• Added Khmer to _list.json
• Updated _groups.json to include Khmer in Asian languages group

Checks

• Adding quotes?
  • Make sure to include translations for the quotes in the description (or another comment) so we can verify their content.
• Adding a language or a theme?
  • If is a language, did you edit _list.json, _groups.json and add languages.json?
  • If is a theme, did you add the theme.css?
    • Also please add a screenshot of the theme, it would be extra awesome if you do so!
• Check if any open issues are related to this PR; if so, be sure to tag them below.
• Make sure the PR title follows the Conventional Commits standard. (https://www.conventionalcommits.org for more info)
• Make sure to include your GitHub username prefixed with @ inside parentheses at the end of the PR title.

Closes #

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[github-actions]

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[Miodec]

Thanks