Add `--default-platform` option to `package js` #1777

Job	Run time
format	50s
Build for native target (macos-15, Xcode_16.4)	40s
check-bridgejs-generated	5m 31s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.1-release/ubuntu2204/swift-6.1-R...	4m 55s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	5m 1s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.0.2-release/ubuntu2204/swift-6.0...	4m 20s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	5m 58s
build-examples	19m 32s
deploy-examples	0s
	46m 47s

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