test(demo): add go/types Scope.Insert demo

Add a demo program to demonstrate the fix for cross-package interface
private method calls. This demo uses go/types.Scope.Insert which has
private methods in the go/types package, making it a good test case
for the bug fix in PR #1371.

Generated with [codeagent](https://github.com/qbox/codeagent)
Co-authored-by: luoliwoshang <luoliwoshang@users.noreply.github.com>

_demo/go/gotypesissue/main.go

@@ -0,0 +1,32 @@
+package main
+
+import (
+	"go/token"
+	"go/types"
+)
+
+func main() {
+	pkg := types.NewPackage("example", "example")
+	scope := pkg.Scope()
+
+	obj := types.NewVar(token.NoPos, pkg, "testVar", types.Typ[types.Int])
+
+	insertedObj := scope.Insert(obj)
+	if insertedObj != nil {
+		println("ERROR: Variable already exists in scope")
+		return
+	}
+
+	lookup := scope.Lookup("testVar")
+	if lookup == nil {
+		println("ERROR: Failed to lookup variable")
+		return
+	}
+
+	if lookup.Name() != "testVar" {
+		println("ERROR: Wrong variable name")
+		return
+	}
+
+	println("SUCCESS: Scope.Insert and Lookup work correctly")
+}