[Parser] Add 'atContextualKeywordPrefixedSyntax' method #107

Job	Run time
Run tests using swift-syntax-dev-utils (Windows)	49m 59s
Run tests using swift-syntax-dev-utils (Linux)	25m 2s
Validate generated code	2m 56s
Test / Windows (5.9 - windows-2022)	18m 57s
Test / Linux (5.9 - jammy)	4m 1s
Test / Windows (6.0 - windows-2022)	26m 23s
Test / Linux (5.10 - jammy)	3m 50s
Test / Windows (nightly - windows-2019)	15m 20s
Test / Linux (6.0 - jammy)	4m 16s
Test / Windows (nightly-6.1 - windows-2019)	14m 52s
Test / Linux (nightly-main - jammy)	4m 57s
Test / Linux (nightly-6.1 - jammy)	4m 47s
Soundness / API breakage check	0s
Soundness / Broken symlinks check	3s
Soundness / Documentation check	4m 51s
Soundness / Format check	37s
Soundness / License headers check	11s
Soundness / Python lint check	6s
Soundness / Unacceptable language check	5s
Soundness / Shell check	37s
Soundness / YAML lint check	3s
	3h 1m 53s

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