Feature request: vectorized phi()

Stan has a vectorized $\Phi(x)$ (standard cumulative distribution function) but lacks a corresponding vectorized $\varphi(x)$ (standard normal density). You can't get this using std_normal_lpdf(), as it sums the log density values; I'm asking for something that returns a vectorized result.

One use case for a vectorized $\Phi(x)$ is in computing a quasi-hinge function based on $\Phi(x)$:

$h(x) = x \Phi(x) + \varphi(x)$.

This function has range $(0,\infty)$ with $h(x) \to 0$ as $x\to -\infty$, $h(x)/x \to 1$ as $x\to \infty$, and $h'(x) = \Phi(x)$.

Start by reading the existing vectorized Phi() implementation and std_normal_lpdf(), focusing on how vectorized values and summed log densities differ. Define done as a vectorized varphi() that returns one standard-normal density value per input rather than a summed log density, and compare its behavior with the requested quasi-hinge use case.