Add a top-level `.gitignore` #748

Job	Run time
build (ubuntu, stable, native_assets_cli, false)	1m 53s
build (ubuntu, stable, native_toolchain_c, false)	1m 53s
build (ubuntu, dev, native_assets_builder, false)	4m 42s
build (ubuntu, dev, native_assets_cli, false)	2m 20s
build (ubuntu, dev, native_toolchain_c, false)	1m 46s
build (macos, stable, native_assets_cli, false)	1m 53s
build (macos, stable, native_toolchain_c, false)	3m 29s
build (macos, dev, native_assets_builder, false)	4m 46s
build (macos, dev, native_assets_cli, false)	2m 44s
build (macos, dev, native_toolchain_c, false)	2m 29s
build (windows, 3.3.0, native_assets_cli, false)	2m 28s
build (windows, 3.3.0, native_toolchain_c, false)	3m 21s
build (windows, dev, native_assets_builder, false)	6m 3s
build (windows, dev, native_assets_cli, false)	2m 35s
build (windows, dev, native_toolchain_c, false)	4m 6s
coverage-finished	2s
	46m 30s

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