feat: VIEW where NOT NULL

[tianzhou]

Fix #54

[Copilot]

Pull Request Overview

This PR adds support for "IS NOT NULL" (and "IS NULL") expressions in view definitions by implementing proper parsing and comparison functionality for NULL test operations.

• Adds formatNullTest function to handle NULL test expression formatting in the PostgreSQL formatter
• Extends compareExpressions to support NULL test comparisons with a new compareNullTests function
• Updates test data to demonstrate the new functionality with a view containing "IS NOT NULL" conditions

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

File	Description
ir/formatter.go	Adds NULL test expression formatting support with formatNullTest function
internal/diff/view.go	Implements NULL test comparison logic with compareNullTests function
testdata/diff/create_view/add_view/.	Updated test data files to demonstrate NULL test functionality in views

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