Framework case as a gridfinity bin

Case for the framework 13 motherboard in a 7x3x4 gridfinity bin.

This is work in progress, I'll update the model when possible and especially if I find a motherboard to do further tests.

2024-10-30: New file for the bottom. Added holes for air intake. 

2024-11-05: Bug corrected in the bottom.

I've change the external of the case to be a gridfinity bin