docs: add PostgreSQL 18 AST upgrade analysis

[pyramation]

Summary

Adds a comprehensive analysis document (upgrade-18.md) examining the AST changes between PostgreSQL 17 and PostgreSQL 18 based on the libpg_query 17-latest to 18-latest-dev comparison.

Key findings:

• 5 new node types: ReturningExpr, ReturningOption, ReturningClause, ATAlterConstraint
• 1 removed node: SinglePartitionSpec (was empty anyway)
• Major breaking changes: RETURNING clause restructuring (affects all DML statements), RowCompareType replaced by CompareType enum
• New features: SQL:2011 temporal constraints (WITHOUT OVERLAPS, PERIOD), constraint enforceability (ENFORCED/NOT ENFORCED), virtual generated columns
• Migration effort: Medium-High (more than PG16→PG17, less than PG13→PG17)

Review & Testing Checklist for Human

• Verify the RETURNING clause restructuring analysis is accurate (this is flagged as the biggest breaking change)
• Confirm the RowCompareType → CompareType enum change is correctly documented
• Check if any significant AST changes were missed in the libpg_query diff analysis

Notes

This is documentation-only, no code changes. The analysis is intended to help plan the PG18 upgrade work for the parser, deparser, and transformer packages.

Link to Devin run: https://app.devin.ai/sessions/dc6c32ec1699424a99860dc9621b91c1
Requested by: Dan Lynch (@pyramation)

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