vfmt: fix `struct Abc { field fn() ? }`

vlib/v/fmt/tests/struct_with_fn_fields_keep.vv

@@ -0,0 +1,4 @@
+struct FieldsWithOptionalVoidReturnType {
+	f fn () ?
+	g fn () ?
+}

vlib/v/table/table.v

@@ -120,7 +120,9 @@ pub fn (t &Table) fn_type_source_signature(f &Fn) string {
 		}
 	}
 	sig += ')'
-	if f.return_type != void_type {
+	if f.return_type == ovoid_type {
+		sig += ' ?'
+	} else if f.return_type != void_type {
 		return_type_sym := t.get_type_symbol(f.return_type)
 		sig += ' $return_type_sym.name'
 	}

vlib/v/table/types.v

@@ -335,6 +335,7 @@ pub const (
 
 pub const (
 	void_type    = new_type(void_type_idx)
+	ovoid_type   = new_type(void_type_idx).set_flag(.optional) // the return type of `fn () ?`
 	voidptr_type = new_type(voidptr_type_idx)
 	byteptr_type = new_type(byteptr_type_idx)
 	charptr_type = new_type(charptr_type_idx)