Add `--default-platform` option to `package js` #1769

Job	Run time
format	55s
Build for native target (macos-15, Xcode_16.4)	40s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	5m 8s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.1-release/ubuntu2204/swift-6.1-R...	4m 50s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.0.2-release/ubuntu2204/swift-6.0...	4m 35s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	5m 59s
check-bridgejs-generated	5m 36s
build-examples	12m 56s
deploy-examples	0s
	40m 39s

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