Added environment config flags.

These flags are `JAXTYPING_DISABLE` and `JAXTYPING_REMOVE_TYPECHECKER_STACK`.

In addition, have now added warnings when using old-style double-decorator syntax, which also serves to guard against the easy mistake of
```python
@jaxtyped(typechecker)
def foo(...)
```
which actually decorates the `typechecker`, not `foo`.

test/test_threading.py

@@ -39,8 +39,7 @@ class _ErrorableThread(threading.Thread):
 
 
 def test_threading_jaxtyped():
-    @jaxtyped
-    @typechecked
+    @jaxtyped(typechecker=typechecked)
     def add(x: Float[Array, "a b"], y: Float[Array, "a b"]) -> Float[Array, "a b"]:
         return x + y