Adds a new source/blender/asset_system directory and moves asset related files from BKE to it. More asset system related code can follow (e.g. asset indexing, ED_assetlist stuff) but needs further work to untangle it. I also kept BKE_asset.h and asset.cc as is, since they deal with asset DNA data mostly.

Motivation:

• Makes the asset system design more present (term wasn't even used in code before).
• An asset_system directory is quite descriptive (trivial to identify core asset system features) and makes it easy to find asset code.
• Asset system is mostly runtime data, with little relation to other Main/BKE/DNA types.
• There's a lot of stuff in BKE already. It shouldn't be just a dump for all stuff that seems core enough.
• Being its own directly helps us be more mindful about encapsulating the module well, and avoiding dependencies on other modules.
• We can be more free with splitting files here than in BKE.
• In future there might be an asset system BPY module, which would then map quite nicely to the asset_system directory.