Support for transpiling .js files (#9160)

* Remove some `.es6` from comments where it does not matter

* Use a post processor for transpilation

This will allow us to eventually use the directory structure to
transpile rather than the extension.

* FIX: Some errors and clean up in confirm-new-email

It would throw an error if the webauthn element wasn't present.
Also I changed things so that no-module is not explicitly
referenced.

* Remove `no-module`

Instead we allow a magic comment: `// discourse-skip-module` to prevent
the asset pipeline from creating a module.

* DEV: Enable babel transpilation based on directory

If it's in `app/assets/javascripts/dicourse` it will be transpiled
even without the `.es6` extension.

* REFACTOR: Remove Tilt/ES6ModuleTranspiler

spec/lib/discourse_js_processor_spec.rb

@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+require 'discourse_js_processor'
+
+describe DiscourseJsProcessor do
+
+  describe 'should_transpile?' do
+    it "returns false for empty strings" do
+      expect(DiscourseJsProcessor.should_transpile?(nil)).to eq(false)
+      expect(DiscourseJsProcessor.should_transpile?('')).to eq(false)
+    end
+
+    it "returns false for a regular js file" do
+      expect(DiscourseJsProcessor.should_transpile?("file.js")).to eq(false)
+    end
+
+    it "returns true for deprecated .es6 files" do
+      expect(DiscourseJsProcessor.should_transpile?("file.es6")).to eq(true)
+      expect(DiscourseJsProcessor.should_transpile?("file.js.es6")).to eq(true)
+      expect(DiscourseJsProcessor.should_transpile?("file.js.es6.erb")).to eq(true)
+    end
+  end
+
+  describe "skip_module?" do
+    it "returns false for empty strings" do
+      expect(DiscourseJsProcessor.skip_module?(nil)).to eq(false)
+      expect(DiscourseJsProcessor.skip_module?('')).to eq(false)
+    end
+
+    it "returns true if the header is present" do
+      expect(DiscourseJsProcessor.skip_module?("// cool comment\n// discourse-skip-module")).to eq(true)
+    end
+
+    it "returns false if the header is not present" do
+      expect(DiscourseJsProcessor.skip_module?("// just some JS\nconsole.log()")).to eq(false)
+    end
+  end
+end