vbscript: Return error 1045 for non-literal constant expressions.
When a Const declaration uses a non-literal expression (e.g. "Const x = 1 + "a""), Windows parses the full expression and then rejects it with error 1045 ("expected literal constant").
Change ConstDecl to accept Expression instead of ConstExpression, and validate in new_const_decl() that the expression is a literal or negated numeric literal. Remove the now-unused ConstExpression rule.
This fixes both the error code (was generic E_FAIL/16389) and the error character position (now points to the end of the expression).