Add numberOfWeek and support custom #title slot formatting

[dmitry-sazonov-official]

Problem

In week view, vue-cal provides start and end dates for each week, but:

1. There is no direct way to access the ISO week number for the currently displayed week.
2. The #title slot shows only string of dates with the week number, so users cannot display the week number in a custom format.

Many apps require week numbers in the UI (for business calendars, ERP/CRM integration, or reporting) and want to control how the title is rendered, combining start/end dates with the week number.

Proposal
- Add a new computed field (or property) such as numberOfWeek.
- Allow usage in the #title slot so developers can format text freely

Fixes #106