Add .npmignore to reduce package size

**Is your feature request related to a problem? Please describe.**
I'm always frustrated when the npm package size is larger than necessary due to included files that aren't required for production.

**Describe the solution you'd like**
Adding a .npmignore files in each packages to exclude unnecessary files (e.g., __image_snapshots__, docs, configs) from the npm package to reduce its size. Especially, image files in `__image_snapshots__` has unnecessarily big file size and long filename which can cause too long filename error when copying.

**Describe alternatives you've considered**
An alternative would be to manually specify files to include in the package.json, but using .npmignore is more flexible and easier to maintain.

**Additional context**
Reducing the package size will improve installation speed and reduce bandwidth usage for users.