interp: fix fieldName method parsing embedded + generic fields

Fix https://github.com/traefik/yaegi/issues/1571

_test/issue-1571.go

@@ -0,0 +1,20 @@
+package main
+
+type A struct {
+	*B[string]
+}
+
+type B[T any] struct {
+	data T
+}
+
+func main() {
+	_ = &A{
+		B: &B[string]{},
+	}
+
+	println("PASS")
+}
+
+// Output:
+// PASS

interp/type.go

@@ -1230,6 +1230,8 @@ func fieldName(n *node) string {
 		return fieldName(n.child[1])
 	case starExpr:
 		return fieldName(n.child[0])
+	case indexExpr:
+		return fieldName(n.child[0])
 	case identExpr:
 		return n.ident
 	default: