Add custom JVP rule for jnp.sinc

[jakevdp]

The current implementation has incorrect even-ordered derivatives at x=0.0:

import matplotlib.pyplot as plt
import jax.numpy as jnp
from jax import grad, vmap

x = jnp.linspace(-5, 5, 101)
y = vmap(grad(grad(jnp.sinc)))(x)

plt.plot(x, y)

Related to #5039