feat: Add bundled support for Serbea 1.0 templates

[jaredcwhite]

I've been quietly cranking away on the Liquid/Nunjucks/ERB-hybrid template language Serbea for some time now and it's rapidly approaching a 1.0 release.

Once all the latest ERB/components improvements for Bridgetown are done and out of the way, I plan to release Serbea and then pull it is as a dependency. Bridgetown supporting Liquid, ERB, and Serbea out-of-the-box is going to be really powerful and compete favorably with Eleventy (where, to be honest, Nunjucks really rocks).

I know from talking with some folks in the past that Serbea isn't going to be for everyone. That's fine. If you don't like it, you don't have to use it. 😏 But I love it. 😎