Fix TS types and include index.d.ts when publishing to NPM (#16)

* Include index.d.ts when publishing to NPM

* Set correct type for minify in index.d.ts

nodejs/index.d.ts

@@ -22,7 +22,7 @@ export function createConfiguration (options: {
  * @param cfg - Configuration created by {@link createConfiguration}
  * @returns Minified HTML code
  */
-export function minify (src: string, cfg: Cfg): string;
+export function minify (src: string, cfg: Cfg): Buffer;
 
 /**
  * Minifies a {@link Buffer} containing UTF-8 HTML code in place.

nodejs/package.json

@@ -5,7 +5,8 @@
   "main": "index.node",
   "types": "index.d.ts",
   "files": [
-    "postinstall.js"
+    "postinstall.js",
+    "index.d.ts"
   ],
   "scripts": {
     "build": "node-gyp build && shx mv build/Release/index.node index.node",