Introduce a prototype for compiler messages in a separate crate


- Type info about a node
   - Where it is defined
   - What kind of node it is
   - Where are its references
- Autocomplete in some span and given characters
- Running the compiler in structured mode to get diagnostics in a machine-readable format
- Lookup by symbol
- Code tree
- Autofixes ...

Figure out how these messages should be serialised. Should be easily compatible with TS/Deno.
