feat: apply default builder to command() and apply fail() handlers globally

[nexdrew]

Addresses this TODO from #545:

TODO: default to a noop builder in yargs@5.x

When implementing this, I discovered it was necessary to either (a) change the expectations of some tests to move a top-level .fail() handler to a command-specific one or (b) start applying .fail() handlers globally. I opted for the latter due to conversation in #539.

Therefore, this PR solves 2 main problems:

1. Do not require a command to specify a builder in order for validation to apply

   Prior to this, code such as .command('hi').demand('name').global('name') would not enforce the globally required --name option when hi is given on the command line, which has been outlined here, because "running" a command without a builder would actually bypass a reset, reparse, and subsequent validation.

   This PR changes that so a command like .command('hi') is now equivalent to a command like .command('hi', '', {}).

   Note that I have an alternative implementation that applies validation to builder-less commands without the reset or reparse (and means options do not have to be global to be, e.g., required), but I prefer the approach of this PR since it (i) causes all commands to be treated equally as running within a "command context" and (ii) satisfies the previous TODO (and whatever thought process/concerns went with it).

   Accepting this PR means options must be explicitly global (or command-specific) in order to apply to a command.

2. Allow a top-level .fail() handler to handle errors that occur during command execution

   As previously discussed in Better error reporting for subcommands #539.

   This also means that multiple fail handlers at different "command depths" will all be called in depth-first, reverse order. For example, the following code:

   require('yargs')
     .fail((msg, err) => {
       console.log('top-level fail handler 1')
     })
     .command(
       'hi', '',
       (yargs) => {
         yargs
           .fail((msg, err) => console.log('command fail handler 1'))
           .fail((msg, err) => console.log('command fail handler 2'))
       }
     )
     .fail((msg, err) => {
       console.log('top-level fail handler 2')
     })
     .demand('name').global('name').argv

   Would produce this:

   $ node validate-command.js hi
   command fail handler 2
   command fail handler 1
   top-level fail handler 2
   top-level fail handler 1

Note that both of these are BREAKING CHANGES.