feat(bgm): add vite plugin and build integration test

Move the bgm vite plugin into the package so it can be tested in
isolation from the web app. Serve each package as a JSON module,
serializing its maps, and verify resolution through a real vite build
against a self-contained fixture.

packages/bgm/tsconfig.json

@@ -5,5 +5,6 @@
     "rootDir": "./src",
     "lib": ["ES2022", "DOM"]
   },
-  "include": ["src"]
+  "include": ["src"],
+  "exclude": ["src/__fixtures__", "src/**/*.test.ts"]
 }