docs: update READMEs with new APIs and usage examples

[pyramation]

Summary

Updates the README files for both plpgsql-parser and plpgsql-deparser packages to document new APIs added in recent releases (v0.6.x/v0.4.x).

plpgsql-parser additions:

• Return Type Helpers section showing getReturnInfoFromParsedFunction() usage
• Schema Rename Example demonstrating heterogeneous AST transformation (SQL + embedded PL/pgSQL)
• Updated Re-exports section with new exports (getReturnInfo, ReturnInfo, ReturnInfoKind)

plpgsql-deparser additions:

• Return Context section for correct RETURN statement handling with ReturnInfo
• Hydration Utilities section documenting hydratePlpgsqlAst, dehydratePlpgsqlAst, isHydratedExpr, getOriginalQuery

Review & Testing Checklist for Human

• Verify the code examples compile and run correctly (especially the schema rename example which uses multiple imports)
• Confirm the deparseSync(parseResult, {}, returnInfo) signature matches the actual API

Recommended test plan: Copy the schema rename example into a test file and verify it produces the expected output with schema names transformed.

Notes

• Documentation only - no code changes
• Link to Devin run: https://app.devin.ai/sessions/8e1c971e9b194cd9a7dda034c89bd74b
• Requested by: @pyramation

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