Introduce override hooks for suggestions

Related to https://github.com/urfave/cli/pull/1390#discussion_r871398659

app.go

@@ -11,6 +11,8 @@ import (
 	"time"
 )
 
+const didYouMeanTemplate = "Did you mean '%s'?"
+
 var (
 	changeLogURL            = "https://github.com/urfave/cli/blob/main/docs/CHANGELOG.md"
 	appActionDeprecationURL = fmt.Sprintf("%s#deprecated-cli-app-action-signature", changeLogURL)
@@ -18,6 +20,9 @@ var (
 	errInvalidActionType    = NewExitError("ERROR invalid Action type. "+
 		fmt.Sprintf("Must be `func(*Context`)` or `func(*Context) error).  %s", contactSysadmin)+
 		fmt.Sprintf("See %s", appActionDeprecationURL), 2)
+
+	SuggestFlag    SuggestFlagFunc    = suggestFlag
+	SuggestCommand SuggestCommandFunc = suggestCommand
 )
 
 // App is the main structure of a cli application. It is recommended that
@@ -100,6 +105,10 @@ type App struct {
 	didSetup bool
 }
 
+type SuggestFlagFunc func(flags []Flag, provided string, hideHelp bool) string
+
+type SuggestCommandFunc func(commands []*Command, provided string) string
+
 // Tries to find out when this binary was compiled.
 // Returns the current time if it fails to find it.
 func compileTime() time.Time {
@@ -332,6 +341,29 @@ func (a *App) RunContext(ctx context.Context, arguments []string) (err error) {
 	return err
 }
 
+func (a *App) suggestFlagFromError(err error, command string) (string, error) {
+	flag, parseErr := flagFromError(err)
+	if parseErr != nil {
+		return "", err
+	}
+
+	flags := a.Flags
+	if command != "" {
+		cmd := a.Command(command)
+		if cmd == nil {
+			return "", err
+		}
+		flags = cmd.Flags
+	}
+
+	suggestion := SuggestFlag(flags, flag, a.HideHelp)
+	if len(suggestion) == 0 {
+		return "", err
+	}
+
+	return fmt.Sprintf(didYouMeanTemplate+"\n\n", suggestion), nil
+}
+
 // RunAndExitOnError calls .Run() and exits non-zero if an error was returned
 //
 // Deprecated: instead you should return an error that fulfills cli.ExitCoder