Prototype style hooks

[shadowspawn]

This is built on top of #2251 to add simple text style hooks. The style hooks do not get passed enough to do complex coloring, but this allows adding color without needing to subclass Help and Command.

program.usage(
  `${styleText('green', '[options]')} ${styleText('yellow', '[command]')}`,
);
program.configureHelp({
  styleTitle: (str) => styleText('bold', str),
  styleDescription: (str) => styleText('magenta', str),
  styleItemDescription: (str) => styleText('italic', str),
  styleOptionTerm: (str) => styleText('green', str),
  styleSubcommandTerm: (str) => styleText('yellow', str),
});

[shadowspawn]

Lot of simple hooks, but I think it makes simple styling much easier for author. I am going to do some more work in this direction to try and have matching colours in "usage" transparently.

[shadowspawn]

Closing as merged into #2251.
Prototype went well!