PackageToJS: Add hint for missing `.enableExperimentalFeature("Extern")` setting #1493

Job	Run time
Build for native target (macos-15, Xcode_16)	31s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.0.2-release/ubuntu2204/swift-6.0...	4m 34s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.1-release/ubuntu2204/swift-6.1-R...	4m 34s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	5m 57s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	5m 39s
format	34s
	21m 49s

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