Build a Generic AST Builder

[stifskere]

Sub-issue of #3

Objective

Create a reusable and extensible Abstract Syntax Tree (AST) layer to represent SQL constructs (tables, columns, constraints, etc.) across multiple SQL dialects.

Tasks

Stage 1: PostgreSQL + CREATE TABLE (priority)

• Define AST node types relevant for PostgreSQL CREATE TABLE (Table, Column, Constraint).
• Set up the project with generic base types for a rowan-free implementation #12
• Implement node constructors with validation for these types.
• Add basic traversal helpers for tree manipulation.

Stage 2: Full MVP / Other Dialects

• Extend AST to support additional node types (indexes, triggers, etc.).
• Generalize node definitions for MySQL, MariaDB, SQLite.
• Enhance traversal utilities for complex queries.

Note: Stage 2 is lower priority; focus first on Stage 1 to enable usage within Flaky.