Implement KeyValues2 (DMX)

[xPaw]

se2007 leak contains unit tests for dmx.

.NET reference we can use: https://github.com/kristiker/Datamodel.NET

[moofMonkey]

Present in the CS:GO leak as well: https://github.com/SwagSoftware/Kisak-Strike/blob/master/datamodel/datamodel.cpp#L866
Guess it should be far closer to the Source 2 implementation than SE2007, at least strings seem to match.

xref both in Source 2 tools DLLs and in the CS:GO leak: CDataModel: Unable to determine DMX encoding for file %s\n