DEV: Introduce minification and source maps for Theme JS (#18646)

Theme javascript is now minified using Terser, just like our core/plugin JS bundles. This reduces the amount of data sent over the network.

This commit also introduces sourcemaps for theme JS. Browser developer tools will now be able show each source file separately when browsing, and also in backtraces.

For theme test JS, the sourcemap is inlined for simplicity. Network load is not a concern for tests.

spec/lib/discourse_js_processor_spec.rb

@@ -185,4 +185,30 @@ RSpec.describe DiscourseJsProcessor do
       )
     end
   end
+
+  describe "Transpiler#terser" do
+    it "can minify code and provide sourcemaps" do
+      sources = {
+        "multiply.js" => "let multiply = (firstValue, secondValue) => firstValue * secondValue;",
+        "add.js" => "let add = (firstValue, secondValue) => firstValue + secondValue;"
+      }
+
+      result = DiscourseJsProcessor::Transpiler.new.terser(sources, { sourceMap: { includeSources: true } })
+      expect(result.keys).to contain_exactly("code", "map")
+
+      begin
+        # Check the code still works
+        ctx = MiniRacer::Context.new
+        ctx.eval(result["code"])
+        expect(ctx.eval("multiply(2, 3)")).to eq(6)
+        expect(ctx.eval("add(2, 3)")).to eq(5)
+      ensure
+        ctx.dispose
+      end
+
+      map = JSON.parse(result["map"])
+      expect(map["sources"]).to contain_exactly(*sources.keys)
+      expect(map["sourcesContent"]).to contain_exactly(*sources.values)
+    end
+  end
 end