Allow enforcement of trailing slashes on urls and links

[jesseleite]

This PR adds config to enforce trailing slashes on urls and links during SSG generation. This is useful for things like SEO sitemap, RSS feeds, etc. on server hosts that bias towards trailing slashes (ie. Netlify expects trailing slashes for SEO).

Todo

• Add opt-in config
• Wire up config to enforce trailing slashes on ssg:generate only
  • We don't want this always running in the service provider!
• Flesh out test coverage
• Update composer constraint (once Statamic v6 is tagged)

References #65
References statamic/cms#11840