Implement buildStructuredUrl API

[abotteram]

Build an API that builds a URL using a urlStructure and urlParts.

const urlStructure = "https://www.example.com/%category%/%postname%/";
const urlParts = {
    category: [ "transportation", "flying" ],
    postname: "the-cost-of-flying",
};

// Outputs: https://www.example.com/transportation/flying/the-cost-of-flying
buildStructuredUrl( urlStructure, urlParts );

Specs:

• urlStructure must be a string.
• urlParts is a key/value pair object.
• urlParts values can be either a string or an array. In case of an array, the API will join the parts together using /.
• urlParts correspond to their counterparts in the urlStructure surrounded by %, and are inserted in those locations in the urlStructure.