Ship third-party-podspecs in the npm package

Summary:
Hi,

Today I upgraded from RN 0.44 to 0.45.0-rc.0 and noticed I add to include either `CxxBridge` or `BatchedBridge` in the React subspecs in my Podfile to get my project to compile again (https://github.com/facebook/react-native/issues/13010).

Adding `BatchedBridge` works fine. However I wanted to try `CxxBridge` as described in 5aca739cc2 but couldn't do it since the required `third-party-podspecs` folder with `Folly.podspec`, `GLog.podspec` and `DoubleConversion.podspec` hadn't been included in the npm release.

So here is the fix for that.
It should be included in the next 0.45.0-rc release.

Let me know what you think.
Closes https://github.com/facebook/react-native/pull/13922

Differential Revision: D5051477

Pulled By: javache

fbshipit-source-id: e5c527f1ee9c84734d3e3a3d85ec3f1e5d648bef

package.json

@@ -107,7 +107,8 @@
     "React",
     "ReactAndroid",
     "ReactCommon",
-    "README.md"
+    "README.md",
+    "third-party-podspecs"
   ],
   "scripts": {
     "test": "jest",