BeamNG Experimental Advanced Vehicle Building

This is a (very) experimental change to BeamNG allowing for a more advanced vehicle building experience. By making slight edits to jbeam loading scripts, vehicle slots no longer spawn with pre-attached default parts and all slots can now be set to "Empty". For instance, the pickup cab spawns without an interior (no dashboard, seats, etc) and the bed spawns without a tailgate. This also works for engine parts however BeamNG isn't built to handle engines with missing core parts so engines will still run despite missing parts like the intake, exhaust manifold, oil pan and long block.

This change is mostly made to better handle changes to the inventory system of BeamLR which currently allows exploits like new parts being created for free. For instance, removing the tailgate then removing and adding back the bed, a new tailgate is created and the old tailgate stays in the inventory. The new inventory system should properly handle linked parts being removed and since all parts spawn without defaults the previously explained exploit is fixed hopefully allowing for part selling, which is a much requested feature.

The advanced vehicle building script may be released as a standalone mod if there is enough interest. Keep in mind this is very experimental and breaks a lot of stuff, for instance all vanilla configs are missing parts