Add snippet for getMonthDays

snippets/getMonthDays.md

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+---
+title: getMonthDays
+tags: date,intermediate
+---
+
+Get the number of days in the month according to the specified month.
+
+- Use `new Date(year, monthIndex, day)`, set the parameter day to `0` to get last day of the month.
+- Use `Date.prototype.getDate` to get the day of month. 
+
+```js
+const getMonthDays = (year, month) => {
+  const today = new Date();
+  
+  return new Date(year || today.getFullYear(), month || (today.getMonth() + 1), 0).getDate();
+};
+```
+
+```js
+getMonthDays(); // 30
+getMonthDays(2020, 2); // 29
+getMonthDays(2021, 2); // 28
+```