fix: update publish to include directories (#328)

Update package to use `**` file glob.

Tested:

```
npm pack


> @google-cloud/[email protected] prepare /Users/timmerman/Documents/github/googlecloudplatform/functions-framework-nodejs
> npm run compile


> @google-cloud/[email protected] compile /Users/timmerman/Documents/github/googlecloudplatform/functions-framework-nodejs
> tsc -p .

npm notice 
npm notice 📦  @google-cloud/[email protected]
npm notice === Tarball Contents === 
npm notice 11.4kB LICENSE                                                 
npm notice 8.2kB  build/src/middleware/background_event_to_cloudevent.js  
npm notice 6.8kB  build/src/middleware/cloudevent_to_background_event.js  
npm notice 2.5kB  build/src/cloudevents.js                                
npm notice 702B   build/src/functions.js                                  
npm notice 4.1kB  build/src/index.js                                      
npm notice 9.3kB  build/src/invoker.js                                    
npm notice 6.1kB  build/src/loader.js                                     
npm notice 1.7kB  build/src/logger.js                                     
npm notice 2.9kB  build/src/pubsub_middleware.js                          
npm notice 1.9kB  build/src/router.js                                     
npm notice 4.6kB  build/src/server.js                                     
npm notice 1.2kB  build/src/types.js                                      
npm notice 1.5kB  package.json                                            
npm notice 13.1kB CHANGELOG.md                                            
npm notice 9.8kB  README.md                                               
npm notice 1.1kB  build/src/middleware/background_event_to_cloudevent.d.ts
npm notice 911B   build/src/middleware/cloudevent_to_background_event.d.ts
npm notice 1.2kB  build/src/cloudevents.d.ts                              
npm notice 2.7kB  build/src/functions.d.ts                                
npm notice 31B    build/src/index.d.ts                                    
npm notice 1.7kB  build/src/invoker.d.ts                                  
npm notice 422B   build/src/loader.d.ts                                   
npm notice 460B   build/src/logger.d.ts                                   
npm notice 3.0kB  build/src/pubsub_middleware.d.ts                        
npm notice 478B   build/src/router.d.ts                                   
npm notice 503B   build/src/server.d.ts                                   
npm notice 179B   build/src/types.d.ts                                    
npm notice === Tarball Details === 
npm notice name:          @google-cloud/functions-framework          
npm notice version:       1.10.0                                     
npm notice filename:      google-cloud-functions-framework-1.10.0.tgz
npm notice package size:  25.6 kB                                    
npm notice unpacked size: 98.8 kB                                    
npm notice shasum:        bd8c396a221d5cfc18260dde3115b39cb714bcf4   
npm notice integrity:     sha512-4JTv/V9u5PVeG[...]s+cVqdJYiXryQ==   
npm notice total files:   28                                         
npm notice 
google-cloud-functions-framework-1.10.0.tgz
```

Fixes #327

Author: grant

google-cloud-functions-framework-1.10.0.tgz

@@ -27,8 +27,8 @@
     "pretest": "npm run compile"
   },
   "files": [
-    "build/src/*.js",
-    "build/src/*.d.ts"
+    "build/src/**/*.js",
+    "build/src/**/*.d.ts"
   ],
   "bin": {
     "functions-framework": "./build/src/index.js",