eval: Return an empty list from systemlist() when there's no output

This is to emulate the behavior with Vim version of `systemlist()`.
runtime: ProviderCall implementations

- Move the python provider implementation(script_host.py) from the python-client
  repository to this one
- Implement python provider with ProviderCall
- Implement clipboard provider with ProviderCall
- Add runtime scripts for working with external plugins and hosts
- Add tests for the new runtime scripts