Add Package.swift for Swift 4.2

[keith]

This allows Stencil to be compiled with Swift 4.2

We should wait for new releases including kylef/PathKit#56 and kylef/Spectre#37

Also once Swift 4.2 is officially released we can add a CI job for it.

[keith]

It would be nice once this was merged if there was a new release with this for upstream libraries

[djbe]

Do we really need a separate package.swift for this? Can't we just add the supported versions to the existing one?

[keith]

The format for supported versions changed from an array of integers to an array of a new set of enum values with the 4.2 compiler.

[keith]

There's a possibility we could use a compiler version check in the Package.swift to set the right type, I can try that path if you'd like

[djbe]

I don't know if all that's necessary.

With the old format, we can only say [4] right? But can you confirm that, as it stands right now, Stencil doesn't work with Swift 4.2? Or is this just to have the nicer syntax with the enums?

[keith]

Right now if a package depends on Stencil, it will build it with the 4.0 mode of the 4.2 compiler. By adding this file and these supported versions, it would build it with the 4.2 mode of the 4.2 compiler. Users can continue to use it in the 4.0 mode successfully, but at some point it makes sense to support 4.2 like this

[djbe]

Right, but we only just added 4.1 support, partially (see #226). I agree that, when we migrate to 4.2 (if there's anything to migrate), we should update our podspec and Package.swift to reflect this.

For now, I think this is too early.

[djbe]

Whoa, I wouldn't close it, let's keep it for when we're ready. I'm pretty sure that once we figure #226 out, we can do the 4.2 migration.

(I'm not able to reopen this)

[keith]

Ah, I thought you meant longer term. I deleted my fork which is why you can't reopen

[djbe]

Going through #226 again, I'm pretty sure that full Swift 4.1 migration is going to take a while...

Well when we do the migration we'll base the package.swift on this one.