gh-132775: Add _PyCode_GetVarCounts() (gh-133128)

gh-132775: Add _PyCode_GetVarCounts() (gh-133128)

This helper is useful in a variety of ways, including in demonstrating how the different counts relate to one another.

It will be used in a later change to help identify if a function is "stateless", meaning it doesn't have any free vars or globals.

Note that a majority of this change is tests.