Introduce new config option for JS-based Sentry

* Also update environment_variables.md

config/initializers/sentry_javascript.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+class SentryJavascript
+  def self.active?
+    dsn.present? && %w[development test].exclude?(environment)
+  end
+
+  def self.dsn
+    ENV['SENTRY_JAVASCRIPT_DSN']
+  end
+
+  def self.release
+    Sentry.configuration.release
+  end
+
+  def self.environment
+    Sentry.configuration.environment
+  end
+end