New features and new way to handle random weighted items

• Completely changing the way to handle random weighted items
  • continuing to use it as a filename suffix, but now supported from 1-100. The higher the number, the more likely it is to be chosen. By default all files are treated as 100 even if you do nothing
• Adding support for External URL property in metadata
• Allowing to use or not the .JSON file extension for metadata
• Generating rarity file (thanks @philicomus for contributing)
• Cleaning output directory before generating new stuff
• Trim unused assemblies when generating the application
  • in the past, a specific antivirus flagged NFT.net as a false positive during virus scanning when using this approach
• Bug fixes