Add Swift Macro Compatibility Check to CI

[Matejkob]

This PR adds a Swift Macro Compatibility Check GitHub Action to ensure that macro changes remain compatible across different versions of swift-syntax. The action automatically verifies compatibility with major swift-syntax versions (509.0.0, 510.0.0, 600.0.0), running builds and tests. It's also posible to turn on checking agains all versions of swift-syntax.

Key Benefits:

• Automated Compatibility Checks: No need to manually test changes across different swift-syntax versions—this is handled automatically in CI.
• Prevents Breakages: Helps avoid accidentally committing changes that break older versions.

[Matejkob]

A run with all swift-syntax versions took 7 m 46 s. I have changed the argument in the action to run with only the major version–let's see the difference if one of you would be kind enough to approve the workflow again :3

[Matejkob]

Wrapping up: checking against all versions of swift-syntax:

• 509.0.0
• 509.0.1
• 509.0.2
• 509.1.0
• 509.1.1
• 510.0.0
• 510.0.1
• 510.0.2
• 510.0.3
• 600.0.0

for this repository, it takes approximately 8 minutes,

but checking against only major versions:

• 509.0.0
• 510.0.0
• 600.0.0

takes 3 minutes. From my experience, running only against major versions is sufficient since swift-syntax does not introduce significant changes across major versions. @stephencelis @mbrandonw, please let me know which version you prefer.

[stephencelis]

@Matejkob Might be nice to do major versions only. Should the major versions take the latest up to the next major? E.g.:

• 509.1.1
• 510.0.3
• 600.0.0

Or do you think compatibility with the earlier version in a major release is better?

[Matejkob]

@Matejkob Might be nice to do major versions only. Should the major versions take the latest up to the next major? E.g.:

• 509.1.1
• 510.0.3
• 600.0.0

Or do you think compatibility with the earlier version in a major release is better?

At the time I considered this, I concluded that the "major version" should correspond to the version of swift-syntax where a new version marker module was introduced. I believe this approach makes the most sense. For instance, if at some point swift-syntax requires a version marker (empty product) for a non-major version, like 6.0.2, then it can also be added to the list of "major versions" (it should not be called major version anymore).

but if you @stephencelis think otherwise I can change it :>

[Matejkob]

I'm marking it as a draft since I want to make one more change before we land it. But before that let's end discusing the above topic.

[stephencelis]

I think it makes sense, just curious about the thought process 😄 Let us know when it's ready and we'll merge!