Add Explain function for AST to EXPLAIN AST output

Add a new Explain function that takes an ast.Statement and returns
a string in the same format as ClickHouse's EXPLAIN AST output.

The implementation handles:
- SELECT queries with all clauses (FROM, WHERE, GROUP BY, HAVING, ORDER BY, LIMIT, OFFSET)
- Expressions (literals, identifiers, functions, binary/unary operations)
- Table expressions (table identifiers, subqueries, table functions)
- JOINs and ARRAY JOINs
- Window functions with OVER clauses
- CASE/WHEN expressions
- IN/BETWEEN/LIKE expressions
- Lambda expressions

Also adds tests that verify the output matches the explain.txt files
in the parser/testdata directory.

Author: claude