Add `--default-platform` option to `package js` #1767

Job	Run time
format	30s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.0.2-release/ubuntu2204/swift-6.0...	3m 17s
check-bridgejs-generated	5m 37s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	3m 9s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.1-release/ubuntu2204/swift-6.1-R...	3m 23s
Build for native target (macos-15, Xcode_16.4)	57s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	3m 25s
build-examples	12m 3s
deploy-examples	0s
	32m 21s

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