Publish deprecation data

[shawnbot]

I'd like to introduce a new "deprecations" JS/JSON endpoint that both people and robots can read to understand what's both being planned for deprecation and what has been actually removed in each release. For instance:

// deprecations.js
module.exports = {
  planned: {
    // we're planning to deprecate these in the next major version
    selectors: {
      '.btn-purple': 'Purple buttons have been officially deprecated',
      '.btn-orange': 'Orange buttons will be removed because of accessibility issues.',
    },
    // we actually _did_ deprecate these in a recent release 😬
    variables: {
      '$blue-450': 'Please use the $blue-mktg variable instead',
      '$green-450': 'Please use the $green-mktg variable instead'
    }
  },
  // then, when we follow through, we move them into a versioned key:
  '11.0.0': {
    selectors: {
      '.BtnGroup-form': 'Please use the "BtnGroup-parent" class instead.',
      '.avatar-stack': 'Please use the "Avatar-Stack" class instead.'
    }
  }
}

Other types of things we'd want to list will probably include files (when we rename a file) and bundles (for instance, if "marketing-utilities" went away).

I think we would serialize this into JSON (as dist/deprecations.json) when we publish so that it can be read more easily by other tools, such as our forthcoming coverage tool.

[shawnbot]

/cc @zeke who was bitten by the variable deprecation 😊