Implement KeyValues3 (Text)

[yaakov-h]

EDIT: Checklist move to #61

[xPaw]

KV3 text is implemented here, and I think it has no known issues: https://github.com/SteamDatabase/ValveResourceFormat/tree/master/ValveResourceFormat/Serialization/KeyValues

KV3 binary: https://github.com/SteamDatabase/ValveResourceFormat/blob/master/ValveResourceFormat/Resource/ResourceTypes/BinaryKV3.cs

Binary is slightly complicated due to multiple compressions and versions, but luckily all of that has been figured out by now.

The only issue there is the ConvertBinaryOnlyKVType function because binary kv3 has some special types (like bytes for true/false booleans and uint32 in binary which maps to uint64 in the text one).