Use Rails.application.config_for to load the webpacker.yml

[Austio]

This PR adds support for 'erb' compilation of webpacker.yml config files.

Without this change, webpacker.yml will forever live on it's own and be responsible for loading a config file manually and work in unexpected ways, like ERB compilation not working inside of webpacker.yml loading similar to how all other config files work.

After this change, webpacker.yml will now act like every other "rails" yml file that people are used to and things like the below will evaluate properly when the environment is set properly

development: 
  host: <%= ENV.fetch('SOMETHING') { true } %>

Internally, this is a small change, we use the Rails.application.config_for method that is used for all other configuration loads and handles erb compilation.

I looked through the github history and this did not seem like a design decision to intentionally not include compilation.

Closes: #1615