src/vs/base/common/strings.ts/singleLetterHash
export function singleLetterHash(n: number): string {
	const LETTERS_CNT = (CharCode.Z - CharCode.A + 1);

	n = n % (2 * LETTERS_CNT);

	if (n < LETTERS_CNT) {
		return String.fromCharCode(CharCode.a + n);
	}

	return String.fromCharCode(CharCode.A + n - LETTERS_CNT);
}