You can. BTDT, in PCL, because it was the easiest way to add the necessary commands directly into a text file with a text editor in order to get the print just right.
You can. It's called Postscript.
The Print doesn't support it self on source-code, since its physical.
Because no-libre software doesn't allow you to do that.
Some print designers publish the source files they used to create their work, in which case you can do this. Some people call this "Open Design".