feat(daysInMonth): add daysInMonth function

index.ts

@@ -144,4 +144,18 @@ export const DECEMBER = date.DECEMBER; // eslint-disable-line @typescript-eslint
  *
  * Note that there is no year zero in the Gregorian calendar. The year
  * 1 BCE was immediately followed by 1 CE. */
-export const isLeapYear = date.isLeapYear;
+export const isLeapYear = date.isLeapYear;
+
+/** Returns the number of days in the specified month in the specified year.
+ *
+ * @param month - An integer representing the month, in the range `1` (January)
+ *   to `12` (December).
+ *
+ * @param year - An integer representing the year. Positive values represent
+ *   years in the Common Era (CE/AD). For example `2020` represents 2020 CE,
+ *   the year this module was first published to npm. Negative values or zero
+ *   represent years before the Common Era (BCE/BC). Zero represents 1 BCE,
+ *   `-1` represents 2 BCE, `-2` represents 3 BCE, etc. There is no year zero
+ *   in the Gregorian calendar. The year 1 BCE was immediately followed by 1
+ *   CE. */
+export const daysInMonth = date.daysInMonth;