Enable tab completion via CLI to `System.CommandLine` Migration

[antonrademaker]

Dotnet cli supports tab completion, it would be really handy to have support for template parameters / symbols as well?

https://docs.microsoft.com/en-us/dotnet/core/tools/enable-tab-autocomplete

To allow tab completion, dotnet new CLI needs to migrate to System.CommandLine parser.

Subtasks

Done

• [Tab Completion] Create root command #3802
• [Tab Completion] Move --install to new command line parser #3805
• [Tab Completion] Move --uninstall to new command line parser #3806
• [Tab Completion] Move --update-apply & --update-check to new command line parser #3807
• [Tab Completion] Move --search to new command line parser #3804
• [Tab Completion] Move --list to new command line parser #3803
• [Tab Completion] Display curated list for 'dotnet new' using new command line parser #3811
• [Tab Completion] Write tests to ensure that the backwards compatibility will be preserved. #2983
• [Tab Completion] Move --help to new command line parser #3810
• dotnet new help should show choice parameter even when one choice is defined #3263
• [Tab Completion] Move template instantiation to new command line parser #3809
• [Tab completion] final cleanup #4060
• binding arguments for host/telemetry [system-command-line] replaced host and telemetry logger parameters to NewCommand with delegates #4254
• [Tab Completion] Move global options to new command line parser #3812
• add tests for debug: arguments to dotnet.new3.UnitTests #3053
• [Tab Completion] Refactor CommandExamples to use Command definition instead #4034
• [Tab Completion] Integrate dotnet new based on new parser into dotnet CLI. #3814 - done by @baronfel
• [Tab Completion] Move hidden commands to new command line parser #3813

Not started / moved to separate issues

• 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)
    • 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.

Justification

Customer impact

1. better and more intuitive CLI UX, including help
2. enables tab completion
   Engineering impact
3. removes dependency on old parser, requiring workarounds for source-build and separate repo maintanence costs
4. easier cost of adding new commands and options for new features

[rainersigwald]

Another upvote from Twitter: https://twitter.com/buhakmeh/status/1235581946853392386?s=20

cc @donJoseLuis

[donJoseLuis]

we need to find the right place for this issue.