WIP: Generate the theme.json file rather than a custom json object

[bacoords]

The goal here is to use this postcss plugin to generate a theme.json file for Understrap rather than our custom understrap_generate_color_palette(). We could just pass colors, but I'd like to pass as much theme styles as possible, potentially matching all of the typography options, etc and even adding support for gradients/duotones.

• [] Make it opt-in via options so that older sites don't use this
• [] Needs a matching PR on Understrap that passes more BS variables to CSS variables so we can parse them in postcss. See example below
• [] Still needs a lot of working matching the theme.json specs
• [] Causes some minor issues with the layout on the backend, so we need to pass some container sizes or something

Add any values we need to src/sass/theme/_understrap.scss

:root {
	--understrap-spacer: #{$spacer};
	--understrap-border-radius: #{$border-radius};
	--understrap-link-color: #{$link-color};
	--understrap-fs1: #{$h1-font-size};
	--understrap-fs2: #{$h2-font-size};
        --understrap-fs3: #{$h3-font-size};
	--understrap-fs4: #{$h4-font-size};
	--understrap-fs5: #{$h5-font-size};
	--understrap-fs6: #{$h6-font-size};
}