parser: use `?` instead of `?void` errpr

vlib/v/checker/tests/void_optional_err.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/void_optional_err.v:1:16: error: use `?` instead of `?void`
+    1 | fn ret_void() ?void {
+      |                ~~~~
+    2 |     return error('error')
+    3 | }

vlib/v/checker/tests/void_optional_err.vv

@@ -0,0 +1,7 @@
+fn ret_void() ?void {
+	return error('error')
+}
+
+fn main() {
+	_ := ret_void() or { panic('$err') }
+}

vlib/v/parser/parse_type.v

@@ -134,7 +134,11 @@ pub fn (mut p Parser) parse_type() table.Type {
 	}
 	mut typ := table.void_type
 	if p.tok.kind != .lcbr {
+		pos := p.tok.position()
 		typ = p.parse_any_type(language, nr_muls > 0)
+		if typ == table.void_type {
+			p.error_with_pos('use `?` instead of `?void`', pos)
+		}
 	}
 	if is_optional {
 		typ = typ.set_flag(.optional)

vlib/v/tests/option_test.v

@@ -274,7 +274,7 @@ fn test_multi_return_opt() {
 	}
 }
 */
-fn foo() ?void {
+fn foo() ? {
 	return error('something')
 }