[DX] Add a hook_library_alter() to remove unwanted things more easily

This was spun off from @wesruv's comment

As a themer I looooooove that I can do all of these things with libraries. The last 3 are kind of "power-usery", but have solid use-cases, IMO. ... In D8 land you're able to do the following to libraries:

• Add new
• Remove entire library
• Add an asset to an existing library
• Remove a single asset from an existing library
• Override a single asset from an existing library

I think getting the 2nd one done would be a huge step forward, but thought the others might be worth mentioning in case we wanted to think about other potential needs.

It'd be nice to add a library removal function, or library alter. But for now I'll recommend this in the tutorial thing I'm working on.

Related: * https://www.drupal.org/docs/8/theming-drupal-8/adding-stylesheets-css-an... * https://www.drupal.org/node/2201089