feat: add postcode masking type

[bruno]

My apologies, this is my first time contributing here, so I'm not sure what the conventions are, but in our code base we wanted to use a custom masker for postcodes, so I thought I would just contribute this back. It's very simple. Nothing special here, just not sure if this is useful for others.

Added a new 'postcode' masking type that preserves only the first 2 characters. This masking option helps us when working with geographic data where the area code (first part) is needed for analytics while protecting specific location details.

Implemented with basic test and updated documentation with the help of an LLM (Claude).