[Feature] `Environment` does not allow filtering

[compnerd]

Is your feature request related to a problem? Please describe.
It is not possible to filter certain environment variables from the inherited environment for the subprocess. With Foundation's Process, you could set the element to nil to unset the variable, but no such equivalent exists for Environment in Subprocess.

Describe the solution you'd like
Either of two (immediately obvious) approaches:

1. permit .updating(["VARIABLE":nil])
2. add .filtering("VARIABLE")

Describe alternatives you've considered
Both of the solutions in the description would be feasible. Given that the core function for this is not exposed, it is not possible to handle this outside of the package.

Additional context