feat: hook resource sharing

[alestiago]

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<String, dynamic>.

Additional Context

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