Build: Add .npmrc with save-exact=true

This makes commands like `npm install package --save-dev` always get saved
to package.json as a pinned version and not as a `^`-delimited range.

Refs jquery/jquery#6600e2e2b9863dc91601d94541a3808cf3ccc90c

.npmrc

@@ -0,0 +1 @@
+save-exact=true