Main branch should have a `SwiftSyntax510` module

[stephencelis]

Description

While it's suggested to use things like #if canImport(SwiftSyntax509) to target specific versions of SwiftSyntax, there's no way to support versions before 509 and after 509 at the moment. A SwiftSyntax510 module should allow libraries to be compatible with the main branch concurrently with 509, though.

Steps to Reproduce

No response

[ahoppen]

Tracked in Apple’s issue tracker as rdar://116922161

[Matejkob]

Shouldn't the main branch include both SwiftSyntax510 and SwiftSyntax511, especially since we already have an official release/5.10 branch?

[ahoppen]

Shouldn't the main branch include both SwiftSyntax510 and SwiftSyntax511, especially since we already have an official release/5.10 branch?

Both should contain SwiftSyntax510. I’m not sure about SwiftSyntax511 because we haven’t named the next release yet. Could be Swift 5.11 or Swift 6.0 and I don’t want to add SwiftSyntax511 only to remove it in favor of SwiftSyntax600.

[Matejkob]

I've taken the liberty of preparing PRs for both the main and release/5.10 branches, where I've introduced a new target: SwiftSyntax510.