Add an API diff workflow

[czechboy0]

In addition to the existing API breakage workflow, it'd be helpful to see the diff of the swiftinterface file (or equivalent for packages that can't build with library evolution enabled) that the PR is making.

When a package has a large diff where most of the code is implementation, it can be difficult to spot the exact API changes it's making.

There are a few ways this could be surfaced to the user:

• Option A: A comment posted by the workflow back on the PR
• Option B: The workflow just printing the diff and the user needing to click on the GitHub check link and reviewing it manually

Either would be fine to start.

Presumably, this would come with some configurability, mainly the list of modules to diff - there might be internal modules that the maintainer might not care about seeing the diff of, and might even cause noise.

[glbrntt]

This would be really helpful to avoid accidental API additions.

Beyond modules to diff, it'd be helpful to configure what access level you care about too.