Function completion 

pub fn completion(source: &str, offset: usize) -> Vec<CompletionItem>

Returns completions appropriate at offset (byte offset) in source.

Uses the parsed AST to determine context:

• Outside all declarations → top-level keywords.
• Inside a datasource or generator block → config key suggestions.
• Inside a model block:
  • After @ → field attribute names.
  • After @@ → model attribute names.
  • Otherwise → scalar types, user-defined model/enum names, and common field attributes as a convenience.