feat: better completions for extern blcoks
This PR refactors `add_keywords` (making it much clearer!) and enhances completion for `extern` blocks.
It is recommended to reviewing the changes in order of the commits:
- The first commit (f3c4dde0a4917a2bac98605cc045eecfb4d69872) doesn’t change any logic but refactors parts of the `add_keywords` function and adds detailed comments.
- The second commit (5dcc1ab649bf8a49cadf006d620871b12f093a2f) improves completion for `extern` kw and extern blocks.