Port Street Spirit to glug

MapLibre Styles are not designed to be directly written, so some pre-processor needs to be used. Previously charites was used, but it had limitations on includes that dictated the project structure instead of grouping more than one layer in a file. Instead, the style has been ported to glug. glug is a Ruby DSL for MapLibre styles. It is less awkward to write and is powerful enough to do what the style needs. Because glug is ruby-based, the ability to lint and format code with rubocop is added. This uses custom rubocop rules that make sense for stylesheets. Currently the Gemfile points at a fork of Glug as it relies on some bugfixes that have not yet been merged upstream. Sprite generation has advanced and spreet is now used to generate spritesheets instead of basemaps/sprites. With the software changes INSTALL.md was split to have separate directions for shortbread. @systemed, could you have a look at the glug? There are no public examples of large stylesheets that I am aware of, so the conventions of this project are starting with a blank slate.