feat: add EXPLAIN plan support with dialect-aware SQL generation (#298)

Adds a comprehensive EXPLAIN statement builder with fluent API for
generating query execution plans across all supported databases.

Key features:
- Explain builder with dialect-specific SQL generation
- Support for PostgreSQL, MySQL, SQLite, DuckDB, Oracle, BigQuery, Spanner
- sql.explain() factory method and SQL.explain() method

Closes #219

Author: cofin

AGENTS.md

@@ -1,5 +1,7 @@
 # Custom SQLglot Dialects
 
+<a id="custom-sqlglot-dialect"></a>
+
 This guide explains how SQLSpec implements and uses custom SQLglot dialects for database-specific SQL features.
 
 ## Overview