language-server: implement completion

[dev-zero]

ok, we probably need a parsing/abstract syntax tree for this, then it could work like:

1. parse until cursor line
2. if the syntax is correct until here we should get the AST/parsing tree (if we are driving the parsing directly we can simply stop, if not by catching an exception)
3. from the parsing we should get the current section in the schema
4. lookup other keys/values