First-class Terser support

[johnnyshields]

This PR ports https://github.com/ahorek/terser-ruby/blob/master/lib/terser/compressor.rb (MIT licensed) to this gem, to add first-class support for Terser in the same manner that Uglifier, YUI, etc. are supported.

Issue raised in Terser gem to notify the maintainers of this port: ahorek/terser-ruby#65

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Motivation

Recently, I replaced the Uglifier with Terser in my Gemfile and Rails config, expecting it to be a drop in replacement:

  # in Gemfile
  group :assets do
    gem 'terser'  # was gem 'uglifier'
    ...

  # in config/environments/*.rb
  config.assets.js_compressor = :terser  # was :uglifier

When I deployed to staging, the process errored during bootstrapping because constant Terser could not be found. In a nutshell, the reason why Uglifier works and Terser does not is because Sprockets::UglifierCompressor exists in Sprockets lib and does not require the uglifier gem to already be loaded, while the Terser equivalent is loaded by a Railtie in the terser gem which does require loading terser gem first.

[johnnyshields]

There is probably a better way to do this line. Need to investigate how Sass achieves a similar thing.