fix(plpgsql-deparser): support nested DECLARE blocks inside FOR loops

[pyramation]

fix(plpgsql-deparser): support nested DECLARE blocks inside FOR loops

Summary

Implements lineno-based scope tracking to correctly assign variables to their enclosing blocks. Previously, variables declared inside a FOR loop body were hoisted to the top-level DECLARE section, which changed semantics (variables should be reinitialized on each loop iteration).

The fix uses line numbers from the AST to determine which variables belong to which blocks:

• Variables with lineno <= topLevelBlockLineno stay in the top-level DECLARE
• Variables with lineno > topLevelBlockLineno are assigned to the nearest nested block whose lineno is greater than the variable's lineno

This fixes the last remaining failing fixture (plpgsql_varprops-13.sql). All 190 fixtures now pass round-trip testing.

Review & Testing Checklist for Human

• Verify the lineno-based algorithm handles edge cases correctly (e.g., multiple nested blocks at same nesting level, deeply nested blocks)
• Test with a real PL/pgSQL function that has nested DECLARE inside a FOR loop to confirm the output is valid syntax
• Check that functions WITHOUT nested DECLARE blocks still work correctly (regression test)

Recommended test plan: Run the round-trip tests locally (npm test in packages/plpgsql-deparser) and manually inspect the output for plpgsql_varprops-13.sql to verify the nested DECLARE appears in the correct position.

Notes

• The algorithm falls back to keeping all variables at top-level when topLevelBlockLineno is undefined (discovered during debugging with big-function.sql)
• Link to Devin run: https://app.devin.ai/sessions/8e1c971e9b194cd9a7dda034c89bd74b
• Requested by: @pyramation

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