trim down contents of tarball

The tarball we publish is essentially the contents of the repository minus `node_modules`. That is it includes config files for TS, Jest, ESLint etc.

Set `paths` in `package.json` to only include `lib` in the tarball. Furthermore, remove `__mock__` directories, as we do `__snapshots__`.

package.json

@@ -3,6 +3,9 @@
   "version": "0.32.0",
   "main": "lib/index.js",
   "types": "lib/index.d.ts",
+  "files": [
+    "lib"
+  ],
   "repository": "github:guardian/cdk",
   "scripts": {
     "build": "tsc",

tsconfig.json

@@ -17,5 +17,5 @@
     "outDir": "lib",
   },
   "include": ["src/**/*"],
-  "exclude": ["node_modules", "test", "src/**/*.test.ts", "src/**/__snapshots__/**"]
+  "exclude": ["node_modules", "test", "src/**/*.test.ts", "src/**/__snapshots__/**", "src/**/__mocks__/**"]
 }