Add Meslo LG & Meslo LG DZ fonts

This is a neat little font set which is missing from the package repository. It is absolutely great for coding.

A few things which need to be addressed:
* I get a bunch of `cannot cp stat` errors. I think these are due to the `forceCopy` in `builder-defs.nix`, and I am not quite sure how to remove them. I read somewhere that these can be safely ignored.
* The two different font sets (LG & LG DZ) need to be downloaded separately, so that's why I have two different `src` sets, from which I copy to another directory, which acts as the base for the `installFonts` phase. I'm not sure whether this is considered good practice.