Add comprehensive GitHub Copilot instructions for JavaScriptKit development #1672

Job	Run time
Build for native target (macos-15, Xcode_16.4)	50s
check-bridgejs-generated	5m 47s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.0.2-release/ubuntu2204/swift-6.0...	4m 15s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.1-release/ubuntu2204/swift-6.1-R...	4m 42s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	6m 2s
build-examples	11m 30s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	6m 35s
format	39s
deploy-examples	-1s
	40m 19s

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