Note: Still wip, I want to test this with SEO Pro and SSG before we actually merge this.

This PR adds the ability to enforce trailing slashes in URLs via URL::enforceTrailingSlashes(), which has been asked for a fair bit.

The idea is to make Statamic normalize trailing slashes in URLs (ie. remove them by default, or add them if the above is configured), while respecting URL query and fragment at the end of any given URL. To opt into this functionality for any URL output, simply wrap the URL with URL::tidy($url).

In fact, all methods on the URL facade should now return tidied output with normalized trailing slashes where relevant.

For more advanced cases where an addon needs to control flow around enforcing/stripping trailing slashes, it can pass a true/false bool into URL::enforceTrailingSlashes($bool) as necessary (see test coverage around this).

References: statamic/ideas#1293
References: statamic/ssg#65
References: statamic/seo-pro#387