Function parse_source

Build PR AST from a PRQL query string. We have this function in `prqlc` rather than in `prqlc-parser` crate since our logging is in `prqlc` and we want to log the LR. (We could split the logging out into a separate crate, but it has dependencies on `prqlc` internals and would be an effort)

(source: &str, source_id: u16)