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Issue: Need an isDate date-only validator

Issue: Need an isDate date-only validator: a task in LegoFlow-SWE (Harbor dataset). A validator.isDate() function is missing from the library. Users need a simple way to validate date strings such as 2018/08/09 or 1988-07-18 , without resorting to full datetime parsing.

The task

A `validator.isDate()` function is missing from the library. Users need a simple way to validate date strings such as `2018/08/09` or `1988-07-18`, without resorting to full datetime parsing.

Part of Lego-X/LegoFlow-SWE.