Allow use of pyre_extensions in pyre playground, use newer typing_extensions

[hauntsaninja]

I wanted to check some aspect of PEP 698. Since pyre is the reference implementation, https://pyre-check.org/play/ seemed like it would be convenient.

However, my attempts to import the override symbol were met with failure.

https://pyre-check.org/play?input=from%20typing%20import%20override%0Afrom%20typing_extensions%20import%20override%0Afrom%20pyre_extensions%20import%20override

from typing import override
from typing_extensions import override
from pyre_extensions import override

 1:0: Undefined import [21]: Could not find a name `override` defined in module `typing`. 
 2:0: Undefined import [21]: Could not find a name `override` defined in module `typing_extensions`. 
 3:0: Undefined import [21]: Could not find a module corresponding to import `pyre_extensions`. 

It would be nice to a) make sure the playground knows about pyre_extensions, b) use a version of pyre that knows that typing_extensions has override,