Rename struct member to struct field

lang/Nub.Lang/StructField.cs

@@ -0,0 +1,10 @@
+using Nub.Lang.Frontend.Parsing;
+
+namespace Nub.Lang;
+
+public class StructField(string name, NubType type, Optional<ExpressionNode> value)
+{
+    public string Name { get; } = name;
+    public NubType Type { get; } = type;
+    public Optional<ExpressionNode> Value { get; } = value;
+}