Add .gitattributes for generated files #15

Job	Run time
Test / Linux (5.8 - jammy)	3m 36s
Test / Windows (${{ matrix.swift_version }} - ${{ ((inputs.enable_windows_docker && contains(matrix.swift_version, 'nightly') && 'windows-2019') \|\| 'windows-2022') }})	0s
Test / Linux (5.9 - jammy)	3m 57s
Test / Linux (5.10 - jammy)	3m 50s
Test / Linux (6.0 - jammy)	3m 59s
Test / Linux (nightly-main - jammy)	4m 58s
Test / Linux (nightly-6.1 - jammy)	4m 42s
Soundness / API breakage check	0s
Soundness / Documentation check	0s
Soundness / Unacceptable language check	0s
Soundness / License headers check	0s
Soundness / Broken symlinks check	3s
Soundness / Format check	0s
Soundness / Shell check	32s
Soundness / YAML lint check	0s
Soundness / Python lint check	5s
	25m 42s

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