feat: .help() API can now enable implicit help command

[nexdrew]

In addition to a flagged option (e.g. --help), .help() now also supports an implicit command (e.g. help), which is enabled by default.

This means any yargs CLI can now enjoy commands such as:

$ cli help
$ cli help command
$ cli command help

No flag needed. 😎

I opted to build the implicit command into the existing .help() API (instead of creating a new method) in order to:

1. reuse the same tokens for the flag and the command
2. enable it by default for most uses of .help()

It can be explicitly disabled by passing false as one of the 3 arguments.

Note that any help option aliases will also be used as an alias for the command, as long as:

1. the alias has a length greater than 1, or
2. all aliases have a length of 1

This should be considered a 5.x breaking change in case existing CLIs use their own help command.