[ty] Very basic Django support (#29642)
## Summary
This PR adds basic tests for Django support in ty, and adds a first
customization point where we now synthesize `.id` and `.pk` attributes
for primary keys on Django models. Django support currently relies on
`django-types`, a fork of `django-stubs` which requires less
special-casing in type checkers.
Example
```py
from django.db import models
class Book(models.Model):
isbn = models.CharField(max_length=13, primary_key=True)
title = models.CharField()
pages = models.IntegerField(null=True)
def _(book: Book):
reveal_type(book.isbn) # str
reveal_type(book.title) # str
reveal_type(book.pages) # int | None (because null=True)
reveal_type(book.pk) # str (because `isbn` is the primary key)
book.id # error: no automatic `id` with a custom primary key
```
## Test Plan
I wrote new Markdown test and also verified all of our assertions
against Django's runtime behavior.
