[bug]: Missing Moonbase Alpha chain definition in the SDK

The docs mention that Moonbase Alpha is supported: https://thirdweb.com/moonbase-alpha 

However, the [`chain-constants`](https://github.com/thirdweb-dev/js/tree/main/packages/thirdweb/src/chains/chain-definitions) don't seem to have a definition for the same.

Happy to make a PR for the same!