runtime: Reimplement python/clipboard providers in vimscript

Clipboard is implemented with platform-specific shell commands, and python is
implemented with the external plugin facility (rpc#* functions). The
script_host.py file(legacy python-vim emulation plugin) was moved/adapted from
the python client repository.
runtime: Remove 'python_setup.vim'
test: Add tests for the rpc#define#* functions
test: Add tests for the python provider

These tests were adapted from the python-client repository
docs: Update to the latest changes in the plugin infrastructure