Fix Asset Unknown Multiple Definitions

[RobertBColton]

This attempts to fix a conflict introduced by #2170 when the TreeNode was given a field named unknown. This causes multiple definitions of the asset_unknown enum constant.

enigma-dev/CompilerSource/compiler/compile.cpp

Line 238 in 462579a

// NOTE: -1 because protobutt doesn't allow index 0 and we're trying to match GM's values

My solution here was to just rename the new one added to TreeNode to unknown_resource for now.

enum AssetType : int {
  asset_any = -2,
  asset_unknown = -1,
  asset_unknown_resource = 998,
  asset_folder = 99,