[feature] Add the ability to dump all commands and parameters to a json file

This was requested of the .NET CLI https://github.com/dotnet/cli/issues/10673. I feel it is needed across enough CLI's to warrant consideration as a feature of System.CommandLine. Key part is marked.

"Regarding the CLI docs published here: https://docs.microsoft.com/en-us/dotnet/core/tools/dotnet?tabs=netcore21

One thing that will help us get docs published easily for CLI reference is if we can _dump the whole list of commands, descriptions, option/mandatory status, parameters list, etc_. This way we can easily diff between releases to see what has changed. It will also allow us to automate the documentation!

It's very troublesome to update the CLI ref which is literally a copy/paste of output from running the commands by hand..."