api: Add vim_input function and mark vim_feedkeys as deferred

The `vim_feedkeys` must be deferred because it can potentially free the buffer
passed to `os_inchar`(which in turns calls `vim_feedkeys` indirectly).

The new `vim_input` function can be used to emulate user input(Since it does not
mess with the typeahead, it is safe to execute without deferring).