Autocomplete hie.yaml

**Is your feature request related to a problem? Please describe.**

I dont write hie.yaml every often, that make me have to open the hie-bios readme to find how to write a valid hie.yaml.

So I'm thinking if possible to support autocomplete for hie.yaml, no context, just keywords completion.

**Describe the solution you'd like**

**Describe alternatives you've considered**

**Additional context**

No files, tests, or entry points are named. Start by locating the VS Code extension's completion handling for hie.yaml and the existing configuration-related code, then determine where keyword suggestions belong. Done means hie.yaml users receive valid keyword completions without requiring context.

Written by the indexing model from the issue text.