Introduce a prototype for compiler messages in a separate crate

[kontheocharis]

• 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.