feat: Support array subscript and slicing syntax

[ajitpratap0]

Description

Add support for array subscript access and slicing operations.

Example SQL

-- Array subscript (single element)
SELECT tags[1] FROM posts;
SELECT matrix[2][3] FROM data;

-- Array slicing
SELECT tags[1:3] FROM posts;
SELECT arr[2:] FROM table_name;  -- From index 2 to end
SELECT arr[:5] FROM table_name;  -- From start to index 5

Current Behavior

Array subscript [n] may work, but slicing [n:m] is not supported.

Expected Behavior

Parser should handle both single subscript and slice range syntax.

Priority

Medium - enhances PostgreSQL array support

Implementation Notes

• Create ArraySubscriptExpression AST node for single subscript
• Create ArraySliceExpression AST node for range slicing
• Handle optional start/end in slice syntax