Expose parser AST statements and expression/codegen types in published TypeScript declarations

[Copilot]

AlaSQL’s published types only exposed AlaSQLAST.compile(...), leaving parser/codegen consumers to cast around undocumented internals (statements, where, toJS). This PR surfaces the existing parser API in types/alasql.d.ts and documents stable expression node shapes already used in practice.

• Type surface: expose parsed statements

  • Adds statements: Statement[] to AlaSQLAST.
  • Introduces a typed Statement shape with where?: ExpressionWrapper to model common parser access patterns.

• Type surface: document expression codegen contract

  • Adds base AlaSQLExpression with:
    • toJS(context: string, tableid: string | number, defcols: AlaSQLDefaultColumns | null): string
  • Captures default-column map as AlaSQLDefaultColumns.

• Type surface: publish stable node shapes

  • Adds explicit node interfaces for:
    • Op (left, op, right/right1/right2)
    • Column (columnid, optional tableid, databaseid)
    • ParamValue (param)
    • NumValue (value)
    • StringValue (value)
    • LogicValue (value)
    • UniOp (op, right)
  • Adds ExpressionNode union over concrete node types.

• Regression coverage for parser consumers

  • Adds test/test2498.js to lock in:
    • parse(...).statements[0].where
    • where.toJS(...) producing executable JS expression
    • expected node-field access for the documented shapes

const ast = alasql.parse('SELECT 1 FROM ? WHERE x < $y');
const where = ast.statements[0].where;
const js = where.toJS('p', '', null);
const fn = new Function('alasql', 'p', 'params', `return Boolean(${js});`);

• Fixes Expose and document the parser AST + toJS() in the published types #2498