Adds a new `source/blender/asset_system` directory and moves asset
related files from BKE to it. More asset 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, thus make sense in BKE.
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.
Checked with some other core devs, consensus seems that this makes
sense.
