api/msgpack-rpc: Implement `channel_close` and expose to vimscript

Simple function for closing a channel by id
provider: Remove bootstrapping code and options

With the `api_spawn` function exposed to vimscript, there's no need to hardcode
bootstrapping code anymore. Now the `has` function will return true if there's a
"live" implementation registered in the provider module(meaning that the
channel implementing the feature must be alive)
runtime: Add script for bootstrapping the python host