Create LESS and SASS files and disjoint files to FiraSans and FiraMono typefaces

[earaujoassis]

An example could be found in this project/repo: there's a font-face descriptor in CSS, LESS, and SASS "syntaxes". Also, I'm proposing to disjoint the files for Fira Sans and Fira Mono; it would give developers the freedom to use (and load) either one, but not both font-faces.

[patrykdesign]

@potch can you comment or execute?