Implement `Date_Column` subclass

[GregoryTravis]

Wired into expressions as well.

First two methods:

• first_of_month -> Date
• end_of_month -> Date

Start with in-memory and get working there.

[enso-bot[bot]]

Greg Travis reports a new STANDUP for today (2025-12-10):

Progress: - Review for text_column left, right, mid

• Date_Column It should be finished by 2025-12-12.

Next Day: Date_Column

[enso-bot[bot]]

Greg Travis reports a new STANDUP for today (2025-12-11):

Progress: - Date_Column first/end_of_month for postgres, sql server, snowflake

• Debugging snowflake tests It should be finished by 2025-12-12.

Next Day: Date_Column

[enso-bot[bot]]

Greg Travis reports a new STANDUP for today (2025-12-12):

Progress: - Text_Column, Date_Column snowflake test failures It should be finished by 2025-12-12.

Next Day: db Equal_Sum and Standard_Deviation