Fix ALTER EXTENSION deparsing issues

[pyramation]

Fix ALTER EXTENSION deparsing and add DRY refactoring for ObjectType

Summary

This PR fixes multiple issues with ALTER EXTENSION statement deparsing and introduces a DRY refactoring for ObjectType keyword mapping:

1. Fixed ALTER EXTENSION UPDATE TO deparsing: The deparser was checking for defname === 'to' but the AST uses defname === 'new_version', causing it to output NEW_VERSION 2.0 instead of UPDATE TO '2.0'. Also added proper string quoting since DefElem context returns unquoted values.

2. Fixed ALTER EXTENSION SET SCHEMA deparsing: Changed output from SCHEMA to SET SCHEMA to match PostgreSQL syntax.

3. Implemented AlterExtensionContentsStmt handler: Added support for ALTER EXTENSION ... ADD/DROP FUNCTION/... statements which were previously unsupported.

4. DRY refactoring: Created centralized getObjectTypeKeyword() method to eliminate duplicate switch statements in AlterOwnerStmt and AlterObjectSchemaStmt. This reduces ~200 lines of duplicated code.

Test results: All 285 test suites pass (655 tests total), including the new misc-missing-types.test.ts that was previously failing.

Review & Testing Checklist for Human

• Verify ALTER EXTENSION UPDATE TO '2.0' produces correctly quoted output - The string quoting logic in DefElem is manually added because DefElem context returns unquoted strings. Test that version strings are properly quoted in the output.
• Spot check the DRY refactoring didn't break edge cases - The centralized getObjectTypeKeyword() is now used by multiple statement types. While CI passes, manually verify a few ALTER ... OWNER TO statements with different object types still work correctly.
• Test ALTER EXTENSION ADD/DROP with different object types - The new AlterExtensionContentsStmt handler assumes action values of 1 (ADD) and -1 (DROP). Verify this works for both ADD and DROP operations with various object types (FUNCTION, TYPE, etc.).

Notes

• The action enum values (1 = ADD, -1 = DROP) in AlterExtensionContentsStmt are hardcoded based on the AST structure. If these values are incorrect, the handler will produce wrong output.
• The getObjectTypeKeyword() method includes a comprehensive mapping of all ObjectType enum values, but some may not be tested by the current fixture set.

Link to Devin run: https://app.devin.ai/sessions/505114984e764debaca3c75b2dab458c
Requested by: Dan Lynch (pyramation@gmail.com) / @pyramation

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