Build User Varaiable Expression in FunctionBuilder (#340)
Build User Varaiable Expression in FunctionBuilder (#340)

Implement UserVariableBuilder in FunctionBuilder to build for custom variable.

This changes solve issues of subtree with user_variable expr_type

Solve this error while building parsed query.
Message: unknown [expr_type] = user_variable in "function subtree
greenlion/PHP-SQL-Parser,greenlion/PHP-SQL-Parser