include new types files in npm package

Author: lobsterkatie

packages/gatsby/.npmignore

@@ -10,3 +10,5 @@
 # Gatsby specific
 !gatsby-browser.js
 !gatsby-node.js
+!gatsby-browser.d.ts
+!gatsby-node.d.ts

packages/gatsby/scripts/prepack.ts

@@ -6,7 +6,7 @@
 import * as fs from 'fs';
 import * as path from 'path';
 
-const PACKAGE_ASSETS = ['gatsby-browser.js', 'gatsby-node.js'];
+const PACKAGE_ASSETS = ['gatsby-browser.js', 'gatsby-browser.d.ts', 'gatsby-node.js', 'gatsby-node.d.ts'];
 
 export function prepack(buildDir: string): boolean {
   // copy package-specific assets to build dir