Add hierarchical stacking

Given the new research progress on "hierarchical stacking", it would be useful to implement this new tool into the loo package.

This new tool needs more user customization: after a few models are fit, the user needs to

1. Run pointwise model evaluation. For IID data, this is highly automated.
2. Specify what variable that stacking weight should depend on. This cannot be automated.
3. Come up with a stacking model. A template may look like https://github.com/yao-yl/hierarchical-stacking-code/blob/main/well_switch/optim.stan. But this model could be more complicated too.
4. Specify hyper prior.

The challenge for putting this package into the present package lies in that

1. Extra dependence on stan.
2. May even need stanglm/brms syntax to write stacking models.

Start by reviewing the loo package and the linked hierarchical-stacking-code template, especially well_switch/optim.stan. Determine how pointwise model evaluation, the user-selected stacking variable, the stacking model, and its hyperprior would fit with the proposed Stan and stanglm/brms dependencies. Done would mean a defined implementation path for this customizable hierarchical stacking tool.