Values returned by prox! function

For a given function $h$, prox! functions from `ShiftedProximalOperators` return the value of the proximal operator while prox! function from `ProximalOperator` return the value of $h$ evaluate in $\mathrm{prox}_h$.

To be consistent, I think that prox! function from `ShiftedProximalOperators` should also return the (shifted) value of $h$.
For exemple, the Moreau envelope in `ProximalOperator` uses the value of $h$ as the return value of the prox function:
```julia
function (f::MoreauEnvelope)(x)
R = eltype(x)
buf = similar(x)
g_prox = prox!(buf, f.g, x, f.lambda)
return g_prox + R(1) / (2 * f.lambda) * norm(buf .- x)^2
end
```
The return value will be incorrect if the function is derived from `ShiftedProximalOperators`.

Compare the prox! implementations in ShiftedProximalOperators and ProximalOperator, then inspect the MoreauEnvelope call shown in the issue. Make the shifted implementation return the shifted value of h consistently with ProximalOperator, and verify that MoreauEnvelope produces the correct value when built from ShiftedProximalOperators.

Written by the indexing model from the issue text.