Update type hints to be PEP 484 compliant

[sibocw]

1. Update warp type hint syntax from

@wp.kernel
def my_kernel(arr: wp.array(dtype=float)):
    ...

to

@wp.kernel
def my_kernel(arr: wp.array[float]):
    ...

which has been supported since warp 1.12.

2. In my own code, use TypeVars for array dimensions instead of hard-coded strings. I.e., before:

def get_joint_angles(self, fly_name: str) -> Float[np.ndarray, "n_jointdofs"]:  # noqa: F821
    ...

After:

from typing import TypeVar

n_jointdofs = TypeVar("n_jointdofs")

def get_joint_angles(self, fly_name: str) -> Float[np.ndarray, "n_jointdofs"]:
    ...

(in reality all dimension typevars are defined in a new utils.typing module.