feat: Support positional parameters ($1, $2) for prepared statements

[ajitpratap0]

Description

Add support for PostgreSQL-style positional parameters in SQL queries.

Example SQL

SELECT * FROM users WHERE id = $1;
SELECT * FROM orders WHERE user_id = $1 AND status = $2;
INSERT INTO logs (message, level) VALUES ($1, $2);
UPDATE products SET price = $1 WHERE id = $2;

Current Behavior

Positional parameters $1, $2, etc. may not be recognized as valid tokens.

Expected Behavior

Parser should tokenize $n as positional parameter placeholders and create appropriate AST nodes.

Priority

High - essential for prepared statements in PostgreSQL

Implementation Notes

• Add TokenTypePositionalParam token type
• Create PositionalParameter AST node with Position field
• Tokenizer should recognize $ followed by digits