QVar<T> class

Represents a value in SQL with proper escaping and formatting.

The QVar class is responsible for converting Dart values into their SQL representation with proper escaping to prevent SQL injection attacks. It handles various data types including strings, numbers, dates, lists, and null values.

Type parameter T can be used for type hints but doesn't affect the conversion logic.

Example usage:

var stringVar = QVar('Hello World'); // 'Hello World'
var numberVar = QVar(42); // 42
var dateVar = QVar(DateTime.now()); // '2023-12-01T10:30:00.000Z'
var listVar = QVar([1, 2, 3]); // (1, 2, 3)
var nullVar = QVar(null); // NULL