Allow shorthand `[T]` for `list[T]`, `{T}` for `set[T]`, and `{K: V}` for `dict[K, V]`

Nobody has claimed this yet.

There's already `(T, U, V)` for `tuple[T, U, V]` and I think what I wrote is all valid syntax, so I think these shorthands are kind of the obvious way to write these types.

```python
xs: [int] = [3, 6, 1]

def f() -> {float}:
return {4.5, 7.2}

def g(d: {str: int}):
print(d)
```

No file, test, or entry point is named. Start by tracing the implementation of the existing `(T, U, V)` tuple shorthand through parsing and type analysis, then determine where the proposed list, set, and dict forms would be handled. Done means the examples in the issue parse and type-check with the intended types, with coverage for valid and invalid uses.

Written by the indexing model from the issue text.