Add utf8 with bom (#2641)

* Change default file encoding

src/Sdk/DTExpressions2/Expressions2/ExpressionParser.cs

@@ -321,7 +321,7 @@ namespace GitHub.DistributedTask.Expressions2
                     context.Operators.Pop();
                 }
                 var functionOperands = PopOperands(context, parameterCount);
-                
+
                 // Node already exists on the operand stack
                 function = (Function)context.Operands.Peek();