Add a factory function to generate the proper paerser object

As an alternative to doing this

```
from tree_hugger.core import PythonParser

pp = PythonParser()
```

We also suggest having something like this

```
from tree_hugger.language_factory import get_parser

pp = get_parser('Python')
```

As most of the parsers expose a very similar API so having a factory makes sense in this regard.

Start by inspecting the parser classes exposed through tree_hugger.core and the proposed tree_hugger.language_factory entry point. Confirm how parser names map to classes, then verify that get_parser('Python') returns the equivalent parser object and that its API matches direct PythonParser construction.

Written by the indexing model from the issue text.