feat(language): add Khmer (@Earpengai) (#6128)

### 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.
- [x] Adding a language or a theme?
- [x] 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.
- [x] Make sure the PR title follows the Conventional Commits standard.
(https://www.conventionalcommits.org for more info)
- [x] Make sure to include your GitHub username prefixed with @ inside
parentheses at the end of the PR title.

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Co-authored-by: Ear Pengai <[email protected]>

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