[6.2] split the unsafeFlags-using manifest to a Swift 6.2 minimum version

[QuietMisdreavus]

• Explanation: Reinstate the use of unsafeFlags in Package.swift, by increasing the default swift-tools-version to 6.2 and creating a separate manifest for Swift 5.7.
• Scope: Resolves an issue where the fix introduced by [6.2] Guard unsafeFlags with Windows-only conditional to restore SwiftPM compatibility #241 prevented the ability to cross-compile to Windows (or to use the package at all from Windows as a semver-tagged dependency).
• Issues: Can't compile with dependency with unsafe flags for different platform. #244
• Original PRs: split the unsafeFlags-using manifest to a Swift 6.2 minimum version #245
• Risk: Low. Older toolchains are still able to build Swift-Markdown via the additional manifest, and newer toolchains are able to use unsafeFlags in a package dependency.
• Testing: See testing notes on split the unsafeFlags-using manifest to a Swift 6.2 minimum version #245
• Reviewers: @compnerd @d-ronnqvist

[QuietMisdreavus]

@swift-ci Please test