Add support for flag groups

[leodido]

Describe the feature or problem you’d like to solve

We wanna a way to show flag usages grouped into different categories.

Proposed solution

The users will benefit from this feature by having a well-organized set of help messages and manuals.

At the moment we have this help message for the root command:

❯ ./lstn help
Analyze the behavior of your dependencies using listen.dev

Usage:
  lstn [command]

Core Commands:
  in          Inspect the verdicts of your dependencies
  to          Get the verdicts of a package

Additional Commands:
  completion  Generate the autocompletion script for the specified shell
  help        Help about any command
  version     Print out version information

Flags:
      --config string     config file (default is $HOME/.lstn.yaml)
      --endpoint string   the listen.dev endpoint emitting the verdicts (default "https://npm.listen.dev")
      --loglevel string   log level (default "info")
      --timeout int       timeout in seconds (default 60)

Additional help topics:
  lstn config      Details about the ~/.lstn.yaml config file
  lstn environment Which environment variables you can use with lstn
  lstn exit        Details about the lstn exit codes
  lstn manual      A comprehensive reference of all the lstn commands

Use "lstn [command] --help" for more information about a command.

We wanna something more organized, like the following one:

❯ ./lstn help
Analyze the behavior of your dependencies using listen.dev

Usage:
  lstn [command]

Core Commands:
  in          Inspect the verdicts of your dependencies
  to          Get the verdicts of a package

Additional Commands:
  completion  Generate the autocompletion script for the specified shell
  help        Help about any command
  version     Print out version information

Flags:
      --config string     config file (default is $HOME/.lstn.yaml)

Config Flags:
      --endpoint string   the listen.dev endpoint emitting the verdicts (default "https://npm.listen.dev")
  -h, --help              help for lstn
      --loglevel string   log level (default "info")
      --timeout int       timeout in seconds (default 60)

Additional help topics:
  lstn config      Details about the ~/.lstn.yaml config file
  lstn environment Which environment variables you can use with lstn
  lstn exit        Details about the lstn exit codes
  lstn manual      A comprehensive reference of all the lstn commands

Use "lstn [command] --help" for more information about a command.

Of course, this feature must apply to all the child commands too.

Additional context

This feature is not currently supported by Cobra (see spf13/cobra#1327)