extensions/typescript-language-features/src/utils/snippetForFunctionCall.ts/appendJoinedPlaceholders
function appendJoinedPlaceholders(
	snippet: vscode.SnippetString,
	parts: ReadonlyArray<Proto.SymbolDisplayPart>,
	joiner: string
) {
	for (let i = 0; i < parts.length; ++i) {
		const paramterPart = parts[i];
		snippet.appendPlaceholder(paramterPart.text);
		if (i !== parts.length - 1) {
			snippet.appendText(joiner);
		}
	}
}