feat!: add Tableau.MDExExtension

[mhanberg]

This changes the MDEx integration from being a converter to being its
own extension.

The main goal of this is to make mdex an optional dependency for
Tableau, so a Tableau release is not required when mdex updates or makes
breaking changes.

• Adds a convert callback to Tableau.Extension
• Adds a Tableau.Converter.convert function for extensions to use
• Post and Page extensions now will copy all files in the respective
  directory, and will convert them if there is a matching converter for
  that extension. Frontmatter is still required.

Breaking Changes

• mdex is an optional dependency, so you must add it to your deps yourself.
• If you use the lumis syntax highlighter with mdex, you must also add that as a dep yourself.
• MDex and converter options are now set by their extension module
  instead of the :config key.
• The contract for Tableau.Converter.convert now just takes content
  and opts.
• Tableau.Converter is no longer a behaviour.
• Converters now are extensions that implement the convert callback.
• Tableau.markdown is removed. Use Tableau.Converter instead.

[mhanberg]

@halostatue thank you for your earlier updates to mdex, but I've decided to decouple tableau from mdex a bit more. If you get some time, could you try out this branch? I believe I've called out all the important breaking changes, but I should probably also put in some migration tips "change this to this" etc.

edit: you might also have to install the tableau_dev_server package from github as i havent published that yet i dont think.