URL constructor as standalone module

[assaf]

• [ ] Spin URL constructor to own module
• [x] URL constructor: cache path to regexp and list of path parameters
• [x] URL constructor: add query parameters for all values that have no path parameter
• [x] url.baseUrl to set the base URL, otherwise all URLs are relative
• [x] url.rootDir to set the base path for url.self, otherwise error
• [x] Multiple variadic path parameters allowed
• [ ] Update documentation accordingly

Do we want any compatibility with https://developer.mozilla.org/en-US/docs/Web/API/URLPattern?