fix: report a repeated argument in the prql query header

[prql-bot]

A prql query header that repeats an argument now fails with duplicate query definition arguments instead of quietly using the last value. Before this, query_def collected the header's arguments into a HashMap, so each repeat overwrote the earlier value. This compiles with Postgres's LIMIT, not MSSQL's TOP:

$ printf 'prql target:sql.mssql target:sql.postgres\nfrom t\ntake 3\n' | prqlc compile
SELECT
  *
FROM
  t
LIMIT
  3

The same applies to version: in prql version:"0.1" version:"^0.13" the 0.1 requirement, which the compiler would otherwise reject, was discarded without a message.

The new error points at the header span and names every repeated argument, matching the existing unknown query definition arguments diagnostic next to it. The regression test query_def_duplicate_argument in prqlc-parser/src/parser/stmt.rs fails without the check and passes with it. Locally, cargo test -p prqlc-parser and cargo clippy -p prqlc-parser --all-targets -- -D warnings both pass.

Found by the nightly survey of prqlc-parser/src/parser/stmt.rs.