Implement new end-to-end parser class

[neiljdo]

This PR implements a new parser class that skips CCGTree derivations when creating pregroup diagrams.

[dimkart]

Let's decide a name for this parser and rename everything appropriately.

[dimkart]

@neiljdo We will also need to make this work with DisCoCircReader which takes a CCGParser now. We can do it in this or in a follow-up PR.

[dimkart]

@neiljdo Based on the discussions, I think it makes sense to proceed with the following after all:

• rename bert.py to parser.py
• move _sentence2pred() method to parser.py, so that the parser module is not that much coupled with the wrapper.
• move pregroup_tree.py from backend to text2diagram module. We can see pregroup trees as a high-level structure analogous to CCG trees, so text2diagram is the right place I think.

[dimkart]

This looks great, let's merge it and open follow-up PRs for the rest. First thing would be to make it work with DisCoCirc I guess.