Make Stable ABI optional

[karosc]

as of right now, the package compiles stable ABI by default. This has caused some problems for a few users downstream who compile the library as part of their own distributions. I propose we make stable ABI opt-in using an environment var or cmake flag rather than opt-out as it currently is.

This should be a modest change in three files:

1. update swmm-toolkit/CMakeLists.txt
2. update pyproject.toml
3. update build env vars to target stable ABI