parser: require anonymous fn to use `_` for unused parameters (#9262)

Fixes a C error with gcc.

vlib/v/parser/fn.v

@@ -544,6 +544,9 @@ fn (mut p Parser) anon_fn() ast.AnonFn {
 	// TODO generics
 	args, _, is_variadic := p.fn_args()
 	for arg in args {
+		if arg.name.len == 0 {
+			p.error_with_pos('use `_` to name an unused parameter', arg.pos)
+		}
 		p.scope.register(ast.Var{
 			name: arg.name
 			typ: arg.typ

vlib/v/parser/tests/anon_unused_param.out

@@ -0,0 +1,4 @@
+vlib/v/parser/tests/anon_unused_param.vv:1:9: error: use `_` to name an unused parameter
+    1 | _ = fn (int){}
+      |         ~~~
+    2 |

vlib/v/parser/tests/anon_unused_param.vv

@@ -0,0 +1,2 @@
+_ = fn (int){}
+