Tools: Add build-styles script

[ryelle]

This is the script used in wporg-news-2021, which builds SCSS to CSS.

Build the CSS for the theme. First run sass to compile down to plain CSS, then run PostCSS to apply autoprefixer. If the --no-rtl flag is passed, that's all. If not, another PostCSS process is run to apply rtlcss and autoprefixer to the Sass output, and save that to the style-rtl.css file.

I'm not sure if this is the best way to share this between projects, we don't actually have a templating script to use these files yet, but I wanted to get it somewhere for re-use — this would be used on projects that don't need to compile javascript (like basic themes).