Initial implementation

[Reinmar]

We can start with support for letter a-c. We'll add more letters later.

[Reinmar]

In ckeditor/ckeditor5-typing#3 and ckeditor/ckeditor5-typing#4 we'll handle more complex cases like composition and autocorrection. In this ticket we'll focus on handle simple typing.

This will be done through our mutation observer which is an interface over the native one, so it gives us already preprocessed mutations.

As it seems now there will be two types of mutations that we need to handle:

1. Simple text change.
2. Whole text node insertion (so a children change).

The first case is very straightforward. The second one will require a bit more work I think.

Another thing that we need to handle is typing over non-collapsed selection. As we don't want to let the browser delete content when inserting the letter, we need to handle this by ourselves. The plan is to make a list of safe keys which shouldn't modify the content and do nothing when those happen. The rest of the keys will trigger deleting the contents... and let's hope that we can make this heuristic precise enough.