chore(vue3): refactor storage of pikaday in datepicker component

What does this MR do?

Vue3 reactivity allows tracking any properties, not just declared on data, etc. so referencing this.calendar in our datepicker component will trigger warning.

The simplest way will be using this.$options.calendar instead, however this.$options is read-only in Vue3, so we need to put calendar inside an object, so now this is this.$options.pikaday.calendar

This is forward-compatible change to support @vue/compat #1981 (closed)

Does this MR meet the acceptance criteria?

Conformity

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