[TOOLS] func_detail_blocker support in VBSP

[newgreenshoot]

func_detail_blocker is a brush entity from L4D that prevents detail sprites and props from being placed by the compiler inside of its volume, allowing finer-grained control over their placement, without needing to use a separate material or cut up displacements.

Unfortunately, we don't have the map compiler code from post-Orange Box branches, but this is a very simple function, and it was reimplemented by wouterpleizier in 2014. To my knowledge, this preceded any leaks that included map compiler code new enough to support func_detail_blocker, and a quick comparison of it against the leaked code suggests that it has no connection, so it should be clean.

The relevant commit can be found here, it would be trivial to cherrypick: wouterpleizier@804ff1c

Unlike L4D's implementation of the brush entity, it doesn't get written to the BSP and is only used during map compile, so the entity shouldn't need to be linked in server code?

This needs a corresponding FGD update as well. It doesn't take any KVs or I/O, just an empty @solidClass.