feat: hook resource sharing

**Description**

Sometimes, it is useful to resources between the `pre_gen` and `post_gen` hook. Say for example, you perform an expensive computation in the `pre_gen` hook to derive some variables to be used by your template, and would like to reuse the output of such expensive computation in the `post_gen` hook.

This is already possible by defining a variable in the `pre_gen` hook context, to be later consumed by the `post_gen` hook. However, this exposes a new variable in the template unnecessarily. In addition, there is some casting to do since the hook context variables type is `Map`.

**Additional Context**

- Private variables, like `_my_var`(leading `_`) might help avoid exposing new variables unnecessarily.