Update .npmignore and .gitignore

[friederbluemle]

Use the files array in package.json to explicitly include files to be distributed.
This leads to a small size reduction of the archive (by excluding unnecessary files like .eslintrc, .flowconfig etc), as well as removes any possibility of accidentally including build output.

Also set type declarations to the already existing file index.d.ts:

"types": "index.d.ts"