extensions/typescript-language-features/web/webServer.ts/parseServerMode
function parseServerMode(): ts.LanguageServiceMode | string | undefined {
	const mode = findArgument('--serverMode');
	if (!mode) { return undefined; }
	switch (mode.toLowerCase()) {
		case 'partialsemantic':
			return ts.LanguageServiceMode.PartialSemantic;
		case 'syntactic':
			return ts.LanguageServiceMode.Syntactic;
		default:
			return mode;
	}
}