Fix ProcessManager signatures to be consistent with LocalProcessManager (#58)

When `LocalProcessManager.run`, `.runSync`, and `.start` were
migrated for null-safety, the base `ProcessManager` interface was not
updated.  This led to an inconsistency where the interface
unnecessarily required non-null versions of `workingDirectory` and
`environment`, which creates extra headache for clients.  This change
should be safe since it makes the base interface less restrictive.

Additionally, `LocalProcessManager` takes a covariant `List<Object>`
where the base interface takes `List<dynamic>`.  I don't see a need
for the interface to take `List<dynamic>` (which could accept `null`)
so change that too.  This change is less safe, although in practice
anything that attempted to pass `null` would already crash if using
the `LocalProcessManager` implementation anyway.

Fixes https://github.com/google/process.dart/issues/56.