web: Build a separate plugin polyfill target file.

A future commit will add a mechanism to inject this into `content.ts` in lieu of the old copy of this file.

web/packages/extension/src/plugin-polyfill.ts

@@ -0,0 +1,8 @@
+// This file is compiled and then injected into content.ts's compiled form.
+
+import {
+    installPlugin,
+    FLASH_PLUGIN,
+} from "ruffle-core/pkg/plugin-polyfill.js";
+
+installPlugin(FLASH_PLUGIN);

web/packages/extension/webpack.config.js

@@ -86,6 +86,7 @@ module.exports = (env, _argv) => {
             ruffle: "./src/ruffle.ts",
             background: "./src/background.ts",
             player: "./src/player.ts",
+            pluginPolyfill: "./src/plugin-polyfill.ts",
         },
         output: {
             path: path.resolve(__dirname, "assets/dist/"),