feat(extensions)!: pre_render phase

[mhanberg]

The pre_render phase is ran after the pre_build phase but before the
graph is traversed and every page is rendered to HTML.

Extensions are also now able to mount to multiple phases in the build
cycle. This allows extensions like the Page and Post extension to
separate the construction of their pages/posts into separate phases.

Doing so means that other extensions are able to modify the data
structures in the token before they are rendered. For example, you can
implement a Table of Contents extension that loops through each post,
creates a TOC structure from markdown headings, then put that structure
into the post data in the token.

This data is then in the token and available to use in layouts.

BREAKING CHANGE: Extensions no longer implement a run/1 callback. They
can implement a callback named after the build phase: pre_build/1,
pre_render/1, etc

fix!: correctly slugify permalinks

BREAKING CHANGE: This switches to using the slugify
package for creating slugs. This leads to better slugs, but also means
your existing page's permalinks might change, which might break
backlinks. When upgrading, please check for permalink changes so that
you can set up the proper redirects. You can easily do this by building your site before the update, running find _site/ | sort > before.txt, then updating, deleting and rebuilding your site, then run find _site/ | sort > after.txt and then comparing the files to look for differences.