Assembly definition tweaks, VPM manifest additions
- Disabled GUID usage on the assembly definition references
- Added define constrains, VRC_SDK_VRCSDK3 and UDON (doesn't compile UdonExplorer's scripts if the keywords are not defined)
- Added VPM manifest info
NOTE: If errors appear in console about duplicate assemblies and/or conflicting GUIDs after importing the new version, delete the old "UdonExplorerAssembly.asmdef" or do a clean import