Add a `SendableMetatype` conformance to `Macro` #210

Job	Run time
Validate generated code	2m 46s
Run tests using swift-syntax-dev-utils (Linux)	1m 51s
Run tests using swift-syntax-dev-utils (Windows)	4m 14s
Test / Linux (5.9 - jammy)	1m 29s
Test / Linux (5.10 - jammy)	1m 19s
Test / Windows (5.9 - windows-2022)	4m 18s
Test / macOS (Xcode ${{ matrix.xcode_version }} - ${{ matrix.os_version }} - ${{ matrix.arch }})	0s
Test / Windows (6.0 - windows-2022)	4m 15s
Test / Linux (6.0 - jammy)	1m 33s
Test / Windows (6.1 - windows-2022)	4m 18s
Test / Windows (nightly - windows-2019)	4m 21s
Test / Linux (6.1 - jammy)	1m 33s
Test / Windows (nightly-6.1 - windows-2019)	4m 19s
Test / Linux (nightly-main - jammy)	4m 15s
Test / Linux (nightly-6.1 - jammy)	1m 57s
Soundness / API breakage check	0s
Soundness / Broken symlinks check	4s
Soundness / Documentation check	3m 57s
Soundness / License headers check	12s
Soundness / Format check	38s
Soundness / Python lint check	7s
Soundness / Shell check	36s
Soundness / Unacceptable language check	6s
Soundness / YAML lint check	4s
	48m 12s

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