feat(language): add xhosa_3k (@aitchz) (#6464)

### Description

Add xhosa_3k

### Checks

- [x] Adding a language or a theme?
- [x] If is a language, did you edit `_list.json`, `_groups.json` and
add `languages.json`?
- [x] 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.

---------

Co-authored-by: Miodec <jack@monkeytype.com>

frontend/static/languages/_groups.json

@@ -531,7 +531,7 @@
   },
   {
     "name": "xhosa",
-    "languages": ["xhosa"]
+    "languages": ["xhosa", "xhosa_3k"]
   },
   {
     "name": "tibetan",

frontend/static/languages/_list.json

@@ -392,6 +392,7 @@
   ,"code_typescript"
   ,"code_odin"
   ,"xhosa"
+  ,"xhosa_3k"
   ,"tibetan"
   ,"tibetan_1k"
   ,"code_cobol"