Support for custom event handler

Support for custom event handler

I am currently using Reedline to develop a console for a custom language. And I am trying to implement the following feature:

• Automatically add indent when enter is hit but validator says input is incomplete.

>>> if 3:
...     a = 5
...     a<-- next line should start here

• Automatic delete 4 spaces when delete is hit. My console treats tab as 4 spaces so I bind Tab key to automatically insert 4 spaces but I can not let delete to behave correspondingly. For example, delete 4 space if there are 4 before the cursor, otherwise delete a single character if there is any.

>>>     <-- delete all 4 previous space here
>>> asbds<-- only delete `s` from buffer

There are also an option that a special event is made for indent handling. But it could be more useful and extensive if user defined handler for ReedlineEvent can be supported.

Start by tracing how ReedlineEvent, key bindings, validation, and the Tab binding are handled. Determine how a user-defined event handler could support indentation on incomplete input and context-sensitive deletion. Done means the handler API supports these behaviors without requiring special built-in indent events.