Add `--default-platform` option to `package js` #1768

Job	Run time
format	1m 7s
check-bridgejs-generated	5m 27s
Build for native target (macos-15, Xcode_16.4)	47s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.0.2-release/ubuntu2204/swift-6.0...	4m 33s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	5m 11s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	5m 53s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.1-release/ubuntu2204/swift-6.1-R...	4m 57s
build-examples	12m 17s
deploy-examples	0s
	40m 12s

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