Support `deleteMyCommands` as well as the (relatively) new parameters `scope` and `language_code`
This MR implements:
- the type
BotCommandScope
astypes::bot_command::Scope
; - the parameters
{SetMyCommands,GetMyCommands}::{scope,language_code}
; - the new method
DeleteMyCommands
.
Part of #66.
Edited by Artem Starikov