refactor: use Pratt's parser for expressions in attributes

This doesn't merge cleanly due to the custom attributes commit that landed but I'm opening a PR anyway for the early review. 

@mlaval I will need your help with rebasing this one on top of your changes. Could you give me a hand with this one when you are available?
I am confused about that change...
I thought that for consistency we wanted to keep _model_ everywhere instead of _value_.

One simple example is that for `<textarea></textarea>` it would not make sens to introduce a _value_ attribute. _model_ would be better. Thus we should use _model_ everywhere...