Add JSONFormatter for PostgreSQL JSON Array Output

[Copilot]

This PR adds a new JSONFormatter transformer that converts SQL queries into PostgreSQL-compatible JSON-returning queries. This implementation follows the pattern from CarbunqleX, enabling developers to easily generate JSON output directly from SQL queries without manual string manipulation.

Features added

• New JSONFormatter class that transforms SELECT queries to return JSON array results
• Support for converting column expressions into JSON key-value pairs
• COALESCE wrapping to handle empty result sets ([] instead of NULL)
• Option to use either jsonb_agg or json_agg functions
• Support for creating nested JSON structures through the groupBy option
• Comprehensive test coverage for various query scenarios
• Documentation in README.md with usage examples

Usage examples

Basic usage:

const query = SelectQueryParser.parse(`
  SELECT id, name, email FROM users WHERE active = true
`);

// Generate JSON-returning SQL
const jsonSql = new JSONFormatter().visit(query);

// Result:
// SELECT COALESCE(jsonb_agg(jsonb_build_object(
//   'id', "id",
//   'name', "name",
//   'email', "email"
// )), '[]') AS result
// FROM "users"
// WHERE "active" = true

Nested JSON with groupBy:

const jsonSql = new JSONFormatter({ 
  groupBy: {
    'users': ['id', 'name', 'email'],
    'orders': ['order_id', 'order_total', 'order_date']
  }
}).visit(query);

// Creates a nested structure with orders as a nested array within each user

This makes it simpler for applications to generate JSON directly from the database, which is particularly useful for API backends that need to return structured JSON data.

Fixes #14.

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