[Parser] Add 'atContextualKeywordPrefixedSyntax' method #108

Job	Run time
Run tests using swift-syntax-dev-utils (Linux)	25m 10s
Run tests using swift-syntax-dev-utils (Windows)	49m 27s
Validate generated code	3m 5s
Test / Linux (5.9 - jammy)	3m 59s
Test / Windows (5.9 - windows-2022)	23m 40s
Test / Linux (5.10 - jammy)	3m 45s
Test / Windows (6.0 - windows-2022)	24m 26s
Test / Linux (6.0 - jammy)	4m 1s
Test / Windows (nightly - windows-2019)	16m 42s
Test / Linux (nightly-main - jammy)	4m 50s
Test / Windows (nightly-6.1 - windows-2019)	15m 27s
Test / Linux (nightly-6.1 - jammy)	4m 48s
Soundness / API breakage check	0s
Soundness / Documentation check	4m 56s
Soundness / Broken symlinks check	5s
Soundness / Format check	42s
Soundness / License headers check	12s
Soundness / Python lint check	8s
Soundness / Shell check	31s
Soundness / Unacceptable language check	6s
Soundness / YAML lint check	6s
	3h 6m 6s

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