parser: handle `a,b += 1,1` (fix #5735) (#5783)

vlib/v/parser/assign.v

@@ -93,6 +93,9 @@ fn (mut p Parser) partial_assign_stmt(left []ast.Expr) ast.Stmt {
 		// a, b = b, a
 		for r in right {
 			has_cross_var = p.check_cross_variables(left, r)
+			if op !in [.assign, .decl_assign] {
+				p.error('unexpected $op.str(), expecting := or = or comma')
+			}
 			if has_cross_var {
 				break
 			}