Add `--default-platform` option to `package js` #1778

Job	Run time
Build for native target (macos-15, Xcode_16.4)	47s
format	30s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	6m 38s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	6m 52s
build-examples	12m 41s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.1-release/ubuntu2204/swift-6.1-R...	5m 2s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.0.2-release/ubuntu2204/swift-6.0...	4m 59s
check-bridgejs-generated	5m 31s
deploy-examples	0s
	43m 0s

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