Make getMonth() return strong string types

I didn't change setMonth(), because that accepts more variants

Author: SLaks

src/hdate.ts

@@ -19,6 +19,7 @@
     along with this program. If not, see <http://www.gnu.org/licenses/>.
  */
 import {
+  MonthName,
   SimpleHebrewDate,
   abs2hebrew,
   daysInMonth,
@@ -324,7 +325,7 @@ export class HDate {
    * const hd = new HDate(new Date(2008, 10, 13)); // 15 Cheshvan 5769
    * hd.getMonthName(); // 'Cheshvan'
    */
-  getMonthName(): string {
+  getMonthName(): MonthName {
     return getMonthName(this.getMonth(), this.getFullYear());
   }
 
@@ -610,7 +611,7 @@ export class HDate {
    * import {HDate, months} from '@hebcal/hdate';
    * HDate.getMonthName(months.CHESHVAN, 5769); // 'Cheshvan'
    */
-  static getMonthName(month: number, year: number): string {
+  static getMonthName(month: number, year: number): MonthName {
     return getMonthName(month, year);
   }
 

src/hdateBase.ts

@@ -50,7 +50,7 @@ export const months = {
   ADAR_II,
 } as const;
 
-const monthNames0: string[] = [
+const monthNames0 = [
   '',
   'Nisan',
   'Iyyar',
@@ -70,11 +70,14 @@ const monthNames0: string[] = [
  * Regular years are index 0 and leap years are index 1.
  * @private
  */
-const monthNames: string[][] = [
-  monthNames0.concat(['Adar', 'Nisan']),
-  monthNames0.concat(['Adar I', 'Adar II', 'Nisan']),
+const monthNames = [
+  [...monthNames0, 'Adar', 'Nisan'],
+  [...monthNames0, 'Adar I', 'Adar II', 'Nisan'],
 ] as const;
 
+/** Transliterated Hebrew month names. */
+export type MonthName = (typeof monthNames)[number][number];
+
 const edCache: Map<number, number> = new Map<number, number>();
 
 const EPOCH = -1373428;
@@ -227,7 +230,7 @@ export function daysInMonth(month: number, year: number): number {
  * @param month Hebrew month (e.g. months.TISHREI)
  * @param year Hebrew year
  */
-export function getMonthName(month: number, year: number): string {
+export function getMonthName(month: number, year: number): MonthName {
   assertNumber(month, 'month');
   assertNumber(year, 'year');
   if (month < 1 || month > 14) {