Add `--default-platform` option to `package js` #1770

Job	Run time
Build for native target (macos-15, Xcode_16.4)	46s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	1m 32s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.1-release/ubuntu2204/swift-6.1-R...	1m 21s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.0.2-release/ubuntu2204/swift-6.0...	1m 17s
check-bridgejs-generated	6m 5s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	1m 22s
build-examples	1m 35s
format	47s
deploy-examples	0s
	14m 45s

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