Add getQueryArg function to url package

[swissspidy]

While working on #10862 I noticed that there's no easy way to retrieve a single query argument from a variable when using the @wordpress/url package.

Since that package already uses qs, it should be trivial to add such a function.

A similar PHP function is also proposed for WordPress core (https://core.trac.wordpress.org/ticket/34699), so we could get feature parity here.

The function could work like this:

import { getQueryArg } from '@wordpress/url';

const url = 'http://wp.org/wp-json/wp/v2/posts?_locale=foo';

const locale = getQueryArg( url, '_locale' ); // 'foo'
const unknown = getQueryArg( url, 'foo' ); // undefined

This way one doesn't have to import qs themselves or use some indexOf or regex trickery to get a query arg or to check for its sole presence.

Happy to work on a PR for that if such a function makes sense.

[swissspidy]

As mentioned in #10862 (comment), this would make it very simple to add a hasQueryArg helper function:

function hasQueryArg( url, var ) {
  return typeof getQueryArg( url, var ) !== 'undefined';
}