feat: Add urlPrefix to sentry configuration

[JoydS]

:scroll: Description

Add UrlPrefix for Source Map

:bulb: Motivation and Context

Required for subpath web app

:green_heart: How did you test it?

Closes https://github.com/getsentry/sentry-dart-plugin/issues/204

:pencil: Checklist

• [x] I reviewed submitted code
• [x] I added tests to verify changes
• [x] I updated the docs if needed
• [x] All tests passing
• [x] No breaking changes

:crystal_ball: Next steps