Potential improvements related to System.CommandLine implementation

[vlada-shubina]

After implementing System.CommandLine, the following low priority improvements were suggested

• use IConsole instead of Reporter - CLI wide
• consider creating TemplatePackageManager.GetTemplates version which uses cache only for tab completion and help
  • consider implementing non async version for GetTemplatePackage based on cache to be used in HandleAmbiguousTemplateGroup and HandleAmbiguousResult methods. At the moment help delegate is not async, also not possible to use async methods in renderer. [system-command-line] error handling for ambiguous results on template instantiation #4227 (comment)
• implement [parse] and [suggest] directives
• fix coloring for error messages for help
• address CommandLineConfiguration.ThrowIfInvalid command-line-api#1582: add corresponding calls to unit tests / re-parsing calls
• consider using middleware for debug options, building environment settings, etc.
• review parsing of template options, some considerations
  • default if option without value should not be parsed / validated at parser level. Instead the parser should just pass null in this case and TemplateCreator should handle this case properly. However if default if option without value is not set for parameter, having no value for option should not be allowed.
  • consider not parsing hex values on parser level. It's a specific format of current generator. Either utilize generator to see if value can be parsed or try to convert any value to hex in format needed by generator and fail if it's not possible.
  • points above should allow to remove custom parse logic for most cases.