Run examples on change

[charlesfrye]

This PR adds a new workflow, Run, that detects changed examples and runs them. This gets us another step closer to full continuous integration of example changes -- it will reduce the number of issues that show up via monitoring (🎉), but we're still missing isolated builds.

To keep testing fast and isolated, all changed examples are executed in parallel job runs (via a jobs matrix output by the changed-files action).

I also centralized the environment setup for the different CI workflows into a single action file, as in the main repo.

FYI, I would not have been able to get this working without the act tool, which runs GitHub Actions locally. Really speeds up debugging!

Type of Change

• Other (changes to the codebase, but not to examples)

[charlesfrye]

Had to revert the updates to typechecking.

mypy<0.981 cannot typecheck any code with positional-only parameters, including modal, due to a bug. In the old way of setting up the type-checking environment, modal was not installed, and so it was not checked.

I tried to update mypy, but versions with the fix are much, much slower. As configured, typechecking was taking about two minutes per example, instead of under a minute total.

So I went back to the old way.

Note that we also can't effectively typecheck any code running inside Modal functions, because the dependencies aren't available to mypy. So we aren't really typechecking much at all here...

[charlesfrye]

merging but still welcome C+C/review to improve!

[thundergolfer]

Was halfway through! haha. Almost got there.

[thundergolfer]

LGTM. thanks for wading through all that YAML 😄