Add marker in scroll bar for most recent message before switching applications


### Discussed in https://github.com/Chatterino/chatterino2/discussions/3221

<div type='discussions-op-text'>

<sup>Originally posted by **MohnJadden7** September  4, 2021</sup>
Add a colored line matching that of the "Draw a line below the most recent message before switching applications" feature to the scrollbar area - similar to highlighted messages.</div>