Fix constructor string parser `change state` algo
Previously, the result was set based on "new state", not based on the existing state. This aligns to Chromium implementation: https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/modules/url_pattern/url_pattern_parser.cc;l=271-275;drc=7bd7edac514e6a13820d9982afaa8a5102bd7688
