Reject opclass options in ON CONFLICT clause

Reject opclass options in ON CONFLICT clause

It's as pointless as ASC/DESC and NULLS FIRST/LAST are, so reject all of
them in the same way.  While at it, normalize the others' error messages
to have less translatable strings.  Add tests for these errors.

Noticed while reviewing recent INSERT ON CONFLICT patches.

Author: Álvaro Herrera <[email protected]>
Reviewed-by: Peter Geoghegan <[email protected]>
Discussion: https://postgr.es/m/202511271516[email protected]