fix: move swift-tools-version to first line of Package.swift

[SalihCanBinboga]

Swift 6.0+ requires the swift-tools-version declaration to be on the very first
line of Package.swift. The current file has a copyright header before it, which
causes Xcode (Swift 6.0+) to fail with:

the manifest is backward-incompatible with Swift < 6.0 because the tools-version
was specified in a subsequent line of the manifest, not the first line.

This moves the // swift-tools-version: 5.9 comment to line 1, before the copyright
header, following the Swift Package Manager specification.

[claude]

Claude Code Review

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[nstosic]

I'm running into the same issue, so I also want to raise urgency on getting this merged and published