[Enhancement][Calendar] Add option for overriding the "Today" button or to use the string from the locale data

[kdikov82]

Currently, if you set different language and locale, the Calendar displays all date related messages in the selected locale, but the "Today" string is set through the messages. The "cldr" packages contain the "Today" string, so we might consider either taking it from the locale data or add an option for custom rendering for the "Today" button.

Example:
https://stackblitz.com/edit/241hm5?file=src%2Fmain.vue,src%2Fes.json

In the example, the locale and the language differ and within the Calendar, only the "Today" string is taken from the messages.
If we decide, we can take the "Today" string from the locale data:

import dateFields from 'cldr-dates-full/main/es/dateFields.json';
console.log(dateFields.main.es.dates.fields.day['relative-type-0']);