Remove unused cross-env dependency

[jaller94]

Current Behavior

´cross-env` is listed a s a dependency, but never used.
https://github.com/palmerhq/tsdx/blob/e942edbf9b74db611db9d4b095c5a742fdd97c99/package.json#L54

Desired Behavior

cross-env does not get listed as a dependency.

Suggested Solution

Remove cross-env from the list of dependencies.

Who does this impact? Who is this for?

All developers

Describe alternatives you've considered

Using cross-env, but the project does not seem to rely on env variables.

Additional context

This project has a scary amount of dependencies. 😨