feat: detect and fix err.String() calls

```diff
-fmt.Errorf("failed for %s with error: %s", "foo", err.Error())
+fmt.Errorf("failed for %s with error: %w", "foo", err)
```

errwrap/errwrap.go

@@ -70,9 +70,21 @@ func run(pass *analysis.Pass) (interface{}, error) {
 		var errIndex int
 		for i, arg := range call.Args {
 			if t := pass.TypesInfo.TypeOf(arg); t != nil {
-				if t.String() == "error" {
+				switch t.String() {
+				case "error":
 					hasError = true
 					errIndex = i
+				case "string":
+					if expr, ok := arg.(*ast.CallExpr); ok {
+						if sel, ok := expr.Fun.(*ast.SelectorExpr); ok {
+							if id, ok := sel.X.(*ast.Ident); ok {
+								if pass.TypesInfo.TypeOf(id).String() == "error" {
+									hasError = true
+									errIndex = i
+								}
+							}
+						}
+					}
 				}
 			}
 		}
@@ -137,6 +149,22 @@ func run(pass *analysis.Pass) (interface{}, error) {
 				}
 			}
 
+			if pass.TypesInfo.TypeOf(call.Args[errIndex]).String() == "string" {
+				arg := call.Args[errIndex]
+				if expr, ok := arg.(*ast.CallExpr); ok {
+					if sel, ok := expr.Fun.(*ast.SelectorExpr); ok {
+						if id, ok := sel.X.(*ast.Ident); ok {
+							// remove the .String call from the error
+							call.Args[errIndex] = &ast.BasicLit{
+								Value:    id.String(),
+								ValuePos: sel.X.Pos(),
+								Kind:     token.STRING,
+							}
+						}
+					}
+				}
+			}
+
 			newExpr := render(pass.Fset, call)
 
 			pass.Report(analysis.Diagnostic{

errwrap/errwrap_test.go

@@ -30,6 +30,10 @@ func Test(t *testing.T) {
 			name: "too many formatting directives",
 			dir:  "d",
 		},
+		{
+			name: "wrap the error.String() with error-wrapping directive",
+			dir:  "e",
+		},
 	} {
 		t.Run(tcase.name, func(t *testing.T) {
 			analysistest.Run(t, testdata, errwrap.Analyzer, tcase.dir)

errwrap/testdata/src/e/e.go

@@ -0,0 +1,11 @@
+package e
+
+import (
+	"errors"
+	"fmt"
+)
+
+func foo() error {
+	err := errors.New("bar!")
+	return fmt.Errorf("failed for %s with error: %s", "foo", err.Error()) // want `call could wrap the error with error-wrapping directive %w`
+}