fix: standardize TIMESTAMPTZ validation

[AyushCoder9]

Description

This PR improves the default value validation logic for the PostgreSQL TIMESTAMPTZ data type.

Changes

• Refactored the checkDefault function for TIMESTAMPTZ in src/data/datatypes.js.
• Ensures that "special values" (like now, today, etc.) are checked first.
• The regex validation for date strings is now more robust and consistent with other timestamp types.

Rationale

Ensures that valid PostgreSQL timestamp-with-timezone strings are correctly identified and prevents false validation failures.

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