[Parser] Add 'atContextualKeywordPrefixedSyntax' method #98

Job	Run time
Run tests using swift-syntax-dev-utils (Linux)	25m 13s
Validate generated code	3m 1s
Run tests using swift-syntax-dev-utils (Windows)	53m 28s
Test / Linux (5.9 - jammy)	3m 52s
Test / Linux (5.10 - jammy)	4m 5s
Test / Linux (6.0 - jammy)	4m 6s
Test / Linux (nightly-main - jammy)	4m 56s
Test / Windows (5.9 - windows-2022)	25m 48s
Test / Linux (nightly-6.1 - jammy)	4m 37s
Test / Windows (6.0 - windows-2022)	22m 43s
Test / Windows (nightly - windows-2019)	17m 37s
Test / Windows (nightly-6.1 - windows-2019)	17m 5s
Soundness / API breakage check	0s
Soundness / Documentation check	5m 19s
Soundness / Unacceptable language check	5s
Soundness / Broken symlinks check	3s
Soundness / Format check	37s
Soundness / License headers check	11s
Soundness / Python lint check	5s
Soundness / YAML lint check	3s
Soundness / Shell check	39s
	3h 13m 33s

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