refactor: codemod ZS2_ → ZL_ across source

[nathandelisle]

Closes #26

replace the legacy ZS2_ prefix with ZL_ by regex across source/headers/docs/CMake; excluded deps/, build/, bld/
verified via git grep that no ZS2_ remain outside excluded dirs
filenames can be addressed in a follow-up if wanted

[Cyan4973]

We don't accept large codemods, as they are difficult / impossible to review.
We also have other non-public requirements for the code to land in our production environment, that can't be possibly tested here. So the codemods will have to be done from within Meta, and exported to github.

We can however review scripts that are meant to do the codemods, and apply them internally.