Create a PHP post processing tool to flip CSS for RTL

One of the requirements for the new theme is to allow customisation from the admin, so we will have to post-process the CSS from PHP, that to leverage the flexibility of SCSS variables.

After generating the CSS a few things will happen, a new one will be to generate (on-demand) the RTL version. This will mimix existing solutions such as rtlcss and css-flip.

This issue is about porting one of the existing tools, agnostic of Moodle codebase, to PHP.

Bootstrap have talked about supporting RTL for years but they aren't there yet. They've also mentioned that they will not have any RTL support for v4.0 but that it may come in a minor release. It goes without saying that we need to support RTL.

This will also have to flip .dir-rtl to .dir-ltr and vice-versa, except when flagged to be ignored.