markused: fix option tracking on sumtype (fix #25025) (#25028)

vlib/json/tests/json_decode_sumtype_option_test.v

@@ -0,0 +1,8 @@
+import json
+
+type Any = string | f32 | bool | ?int
+
+fn test_main() {
+	x := json.decode([]Any, '["hi", -9.8e7, true, null]')!
+	assert dump(x) == [Any('hi'), Any(f32(-9.8e+7)), Any(true), Any('')]
+}

vlib/v/markused/walker.v

@@ -1161,6 +1161,9 @@ pub fn (mut w Walker) mark_by_sym(isym ast.TypeSymbol) {
 					w.features.used_maps++
 					continue
 				}
+				if typ.has_flag(.option) {
+					w.used_option++
+				}
 				w.mark_by_type(typ)
 			}
 		}