1. References -> Manage NuGet Packages -> Install AvalonEditor vào Project
2. Quay lại MainWindow.xaml, thêm 

   xmlns:Avalon="http://icsharpcode.net/sharpdevelop/avalonedit"

   trong thẻ Window

3. Trong thẻ Grid, thêm 

<ScrollViewer VerticalScrollBarVisibility="Visible">
    <Avalon:TextEditor ShowLineNumbers="True"
        Text="">
    </Avalon:TextEditor>
</ScrollViewer>

MainWindow.xaml

<Window x:Class="WpfApp1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp1"
        xmlns:Avalon="http://icsharpcode.net/sharpdevelop/avalonedit"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <ScrollViewer VerticalScrollBarVisibility="Visible">
            <Avalon:TextEditor ShowLineNumbers="True"
                Text="">
            </Avalon:TextEditor>
        </ScrollViewer>
    </Grid>
</Window>