feat: Add PostgreSQL Extension Support

[NFUChen]

Summary

• Automatically detect extensions required by schema objects (e.g., btree_gist for EXCLUDE constraints, citext/hstore for column types) by querying pg_depend catalog dependencies
• Generate CREATE EXTENSION IF NOT EXISTS statements in migration output, ordered before all schema objects to satisfy dependencies
• Support extension diffing (add/drop detection) between old and new schemas

Changes

IR Layer

• ir/ir.go: Added Extensions []string field to IR struct and new Extension struct implementing DiffSource
• ir/inspector.go: New buildExtensions() method that queries pg_catalog.pg_depend to find extensions used by constraints, classes, types, and functions in the target schema (excludes plpgsql and extension-internal objects)

Diff Layer

• internal/diff/diff.go: New DiffTypeExtension, extension comparison logic in GenerateMigration(), and CREATE EXTENSION IF NOT EXISTS DDL generation at the top of generateCreateSQL()

Plan & Dump

• internal/plan/schema_plan.go: Added TypeExtension to object ordering (first in dependency order)
• internal/dump/formatter.go: Added extensions directory to multi-file output and mapping for extension type

Tests

• ir/inspector_extension_test.go: Tests that empty schemas don't detect extensions, and schemas with EXCLUDE constraints correctly detect btree_gist
• testdata/diff/dependency/extension_btree_gist/: New test fixture for btree_gist extension with EXCLUDE USING gist constraint
• Updated fixtures: add_pk, add_column_cross_schema_custom_type to include expected extension output