also disallow assignment to keywords in assignin (bug #46459) * lex.ll (is_keyword): Don't treat "enumeration", "events", "methods", or "properties" as keywords. * oct-parse.in.yy, parse.h: New tests. (Fassignin): Check for assignments to keywords. (octave_base_parser::valid_id_for_assignment): Delete. (octave_base_parser::make_assign_op): Use is_keyword instead of valid_id_for_assignment.