Add 'recipe' project type to Backdrop

This is a spin-off of https://github.com/backdrop-ops/backdropcms.org/issues/46 specifically for proposing a new project type for Backdrop: recipes.

With Backdrop supporting modules for functionality, themes for styling, layouts for, well, layout and install profiles for the initial configuration and setup of a site, I see the addition of a 'recipe' as combining all these separate things into a bundled package of sorts.

Use cases include: - Making it possible to port certain (cough, WordPress) themes to Backdrop - ones that include styling, a custom layout, and features like sliders, etc. (e.g. a theme, a layout and one or more modules). - Providing a full demo site to go along with a tutorial (users install the recipe and get all the modules, content types, themes, etc. setup automatically). - Setting up a new site with things installed and configured the way you (as a developer) normally like them (e.g. simplifying the initial setup of multiple, similar sites).

The reason I like the term 'recipe' (which @klonos originally suggested in the issue linked above) is that 'package' (my original suggestion) implies something that you open to find other things inside, whereas 'recipe' is a bunch of ingredients which, when combined, create something new. I think recipes could be as simple as a single info file, such as:

name = My demo
description = Sets up a Backdrop site for demoing.
backdrop = 1.x # Not sure if this line is necessary...
type = recipe

modules[] = devel
modules[] = webform
modules[] = flexible_layout

themes[] = summer_fun

layouts[] = flexible_template

profiles[] = demo_profile

Or maybe even a JSON file and use it in config somehow... Either way, a simple text file should suffice. And as per info files you should be able to optionally specify versions too.

So modules provide the features/functionality, themes the styling, layouts the layout and profiles the setup (install modules, set theme as default, add blocks to a layout, add custom content types, etc.). Of course all of these would be optional (you could have a recipe of just modules, or a theme and matching layout, etc.).

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Wiki Page Summary Related META Issue: https://github.com/backdrop/backdrop-issues/issues/5240