Changing month and year with Vuex mutation

Let's now add a mutation to the store.

So add a mutations property.

So we need, if you recall, if you want to change the value of the current year at the

current month or any state property then you need a mutation to to do that, and then a

component can call the mutation.

So a mutation is simply just a function.

So let's create one called setCurrentMonth, that way any component which needs to change

the current month can do so, and mutations