Various fixes for sublime_plugin stubs.

[Thom1729]

Based on comments in https://github.com/SublimeText/sublime_lib/pull/160:

• Make CommandInputHandler generic.
• Offer overload for confirm() with event.
• Broaden type for run().

[gwenzek]

Looks good to me :-)

[Thom1729]

Other improvements:

• Remove the unnecessary inheritance clauses.
• meta_info returns dict, not str.
• on_X_command can return None.
• on_query_context returns Optional[bool], not Optional[None].
• Use more specific return type for on_query_completions.