DEV: Fix production sourcemaps with Ember CLI (#16707)

22a7905f restructured how we load Ember CLI assets in production. Unfortunately, it also broke sourcemaps for those assets. This commit fixes that regression via a couple of changes:

- It adds the necessary `.map` paths to `config.assets.precompile`
- It swaps Sprockets' default `SourcemappingUrlProcessor` with an extended version which maintains relative URLs of maps

spec/lib/discourse_sourcemapping_url_processor_spec.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+require 'discourse_sourcemapping_url_processor'
+
+describe DiscourseSourcemappingUrlProcessor do
+  def process(input)
+    env = Sprockets::Environment.new
+    env.context_class.class_eval do
+      def resolve(path, **kargs)
+        "/assets/mapped.js.map"
+      end
+
+      def asset_path(path, options = {})
+        "/assets/mapped-HEXGOESHERE.js.map"
+      end
+    end
+
+    input = { environment: env, data: input, name: 'mapped', filename: 'mapped.js', metadata: {} }
+    DiscourseSourcemappingUrlProcessor.call(input)[:data]
+  end
+
+  it 'maintains relative paths' do
+    output = process "var mapped;\n//# sourceMappingURL=mapped.js.map"
+    expect(output).to eq("var mapped;\n//# sourceMappingURL=mapped-HEXGOESHERE.js.map\n//!\n")
+  end
+
+  it 'uses default behaviour for non-adjacent relative paths' do
+    output = process "var mapped;\n//# sourceMappingURL=/assets/mapped.js.map"
+    expect(output).to eq("var mapped;\n//# sourceMappingURL=/assets/mapped-HEXGOESHERE.js.map\n//!\n")
+  end
+end