Compiling a URLPattern component will generate a 'parts list' which
is used for generating the regular expression that is used for
matching against URLs.
This parts list is also used to generate (through this function) a
pattern string. The pattern string of a URL component is what is
exposed on the USVString getters of the URLPattern class itself.
As an example, the following:
```
let pattern = new URLPattern({ "pathname": "/foo/(.*)*" });
console.log(pattern.pathname);
```
Will log the pattern string of: '/foo/**'.