Add support groups of options and commands in the help

[shadowspawn]

Problem

People would like to be able to group options and commands in the help, and a number of other libraries have support for this.

Requests:

• Show an arbitrary banner message (Feature request) #78
• Request to support for -- parameter separator. #374 (comment)
• Expand help customisation with .addHelpText #1296 (comment)
• Add support for custom help groups #1897

Other libraries:

• Allow grouping of commands yargs/yargs#684
• https://yargs.js.org/docs/#api-reference-groupkeys-groupname
• helpGroup: https://oclif.io/docs/flags
• listGroup: https://github.com/voxpelli/peowly
• https://github.com/75lb/command-line-usage/wiki/How-to-break-the-option-list-up-into-groups

Previous exploration by me: #1908 #1910 #2176 #2248

Solution

Add low-level .helpGroup() on Option and Command, and higher-level .optionsGroup() and .commandsGroup() which can be used in chaining way to specify group title for following options/commands.

See the example file to see them in use.

ChangeLog