Add: Gerber export file function

[kr15h]

Here is a copy/paste from Gerber spec with possible layer file functions that would make sense to implement.

%TF.FileFunction,Legend,Top*%
%TF.FileFunction,Soldermask,Top*%
%TF.FileFunction,Copper,L1,Top*%
%TF.FileFunction,Copper,L2,Inr,Plane*%
%TF.FileFunction,Copper,L3,Inr,Plane*%
%TF.FileFunction,Copper,L4,Bot*%
%TF.FileFunction,Soldermask,Bot*%
%TF.FileFunction,NonPlated,1,4,NPTH,Drill*%
%TF.FileFunction,NonPlated,1,4,NPTH,Rout*%
%TF.FileFunction,Plated,1,4,PTH*%
%TF.FileFunction,Profile,NP*%
%TF.FileFunction,Drillmap*%