Replace os-locale with inline code

[coreyfarrell]

Due to yargs passing spawn: false to os-locale most of the functionality of that module is unused. The following could be used as a drop-in replacement for os-locale:

function osLocale() {
  const locale = env.LC_ALL || env.LC_MESSAGES || env.LANG || env.LANGUAGE || 'en_US';
  return locale.replace(/[.:].*/, '');
}

The benefit is a large drop of production dependencies. Currently a project with yargs as the only dependency installs 53 modules / 1.8MB to node_modules. Without os-locale this becomes 26 modules / 1.1MB. The size reduction is not my primary interest of this issue, the goal is cutting the number of modules that have to be audited in half.

[bcoe]

@coreyfarrell I would happily accept a patch for this, good call.