Implement matrix strategy for macOS job to test against multiple versions of swift-syntax
Use SWIFT_SYNTAX_VERSION environment variable to dictate swift-syntax version per test run
Maintain existing setup for Linux CI (only test "natural" Package.swift swift-syntax version), but update to use swift-actions/setup-swift and Swift 5.10.
Package.swift
swift-syntax version), but update to useswift-actions/setup-swift
and Swift 5.10.