fix: resolve property name conflicts in StructRow.toJSON()[#95]

[Divyanshu-s13]

What's Changed

Fixed a bug in StructRow.toJSON() where field names that match built-in JavaScript object properties (like size, length, toString, etc.) would incorrectly return the built-in property value instead of the actual column data.

The Problem

When a struct had a column named size and you called row.toJSON(), it would return the built-in size property of the object instead of the value from the size column.

The Solution

Modified the toJSON() method in src/row/struct.ts to store the field name in a local variable before using it to access the JSON object. This prevents property name conflicts with built-in object properties.

Closes #95

[kou]

Could you add a test for this?