[ci skip]

We've replaced the sprockets `//= require` directives with ES2015
imports. As a result, the ActionCable javascript can now be compiled
with rollup (like ActiveStorage already is).

- Rename action_cable/index.js.erb -> action_cable/index.js

- Add rake task to generate a javascript module of the ActionCable::INTERNAL ruby hash

  This will allow us to get rid of ERB from the actioncable javascript,
  since it is only used to interpolate ActionCable::INTERNAL.to_json.

- Import INTERNAL directly in ActionCable Connection module

  This is necessary to remove a load-order dependency conflict in the
  rollup-compiled build. Using ActionCable.INTERNAL would result in a
  runtime error:
  ```
  TypeError: Cannot read property 'INTERNAL' of undefined
  ```
  because ActionCable.INTERNAL is not set before the Connection module
  is executed.

  All other ActionCable.* references are executed inside of the body of a
  function, so there is no load-order dependency there.

- Add eslint and eslint-plugin-import devDependencies to actioncable

  These will be used to add a linting setup to actioncable like the one
  in activestorage.

- Add .eslintrc to actioncable

  This lint configuration was copied from activestorage

- Add lint script to actioncable

  This is the same as the lint script in activestorage

- Add babel-core, babel-plugin-external-helpers, and babel-preset-env devDependencies to actioncable

  These will be used to add ES2015 transpilation support to actioncable
  like we have in activestorage.

- Add .babelrc to actioncable

  This configuration was copied from activestorage

- Enable loose mode in ActionCable's babel config

  This generates a smaller bundle when compiled

- Add rollup devDependencies to actioncable

  These will be used to add a modern build pipeline to actioncable like
  the one in activestorage.

- Add rollup config to actioncable

  This is essentially the same as the rollup config from activestorage

- Add prebuild and build scripts to actioncable package

  These scripts were copied from activestorage

- Invoke code generation task as part of actioncable's prebuild script

  This will guarantee that the action_cable/internal.js module is
  available at build time (which is important, because two other modules
  now depend on it).

- Update actioncable package to reference the rollup-compiled files

  Now that we have a fully functional rollup pipeline in actioncable, we
  can use the compiled output in our npm package.

- Remove build section from ActionCable blade config

  Now that rollup is responsible for building ActionCable, we can remove
  that responsibility from Blade.

- Remove assets:compile and assets:verify tasks from ActionCable

  Now that we've added a compiled ActionCable bundle to version control,
  we don't need to compile and verify it at publish-time.

  (We're following the pattern set in ActiveStorage.)

- Include compiled ActionCable javascript bundle in published gem

  This is necessary to maintain support for depending on the ActionCable
  javascript through the Sprockets asset pipeline.

- Add compiled ActionCable bundle to version control

  This mirrors what we do in ActiveStorage, and allows ActionCable to
  continue to be consumed via the sprockets-based asset pipeline when
  using a git source instead of a published version of the gem.

Skipping over 2.4.0 to sidestep the `"symbol_from_string".to_sym.dup` bug.

References #32028

Rails 6 will only support Ruby >= 2.3.

* Depend on websocket-driver >= 0.6.1

This reverts commit 3420a145, reversing
changes made to afb66a5a.

".. with __dir__ we can restore order in the Universe." - by @fxn

Related to 5b8738c2

nio4r 2.0.0 primarily includes new features and bugfixes, with few breaking
changes. The primary reason for bumping the major version is dropping support
for all Ruby versions prior to 2.2.2, so as to match Rails 5.

Full release announcement here:

https://groups.google.com/forum/#!topic/socketry/ZDIUj1ufiJ8

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

Apps that depend on Action Cable don't need Blade for app development,
so we can remove the gem dependency.

We do need Blade for Action Cable dev, so we bundle it in the Gemfile.
Signed-off-by: NJeremy Daer <jeremydaer@gmail.com>

None of the other components use them, so we should be consistent.

(as adapted to use concurrent-ruby / nio4r instead of eventmachine)

- adapter -> pubsub (re)rename internally
- Change variable names to match method names
- Add EventMachine `~> 1.0` as a runtime dependency of ActionCable
- Refactor dependency loading for adapters

This reverts commit d0393fcc, reversing
changes made to 3b7ccadf.

This removes 8 runtime gem dependencies from Rails:

```
Using hitimes 1.2.3
Using timers 4.1.1
Using celluloid-essentials 0.20.5
Using celluloid-extras 0.20.5
Using celluloid-fsm 0.20.5
Using celluloid-pool 0.20.5
Using celluloid-supervision 0.20.5
Using celluloid 0.17.2
```

The issue: https://github.com/celluloid/celluloid/issues/637