Exclude test files from package

[Gudahtt]

The current latest version of this package includes test files (see https://npmfs.com/package/cockatiel/3.1.1/dist/). They aren't necessary to include in the package, so excluding them would be a good opportunity to reduce the package size.

Additionally, these test files are resulting in a security warning from the tool Socket.dev, which analyzes packages for potentially problematic behavior. The warning is about shell access, which is only used in the test files. See here for more information: https://socket.dev/npm/package/cockatiel/overview/3.1.1

Removing the test files would remove that warning.

[Gudahtt]

Thanks!