It is in the AUR:
pamac search positron
positron-ide-devel-bin 2025.12.0.167-1 AUR
A next-generation data science IDE. Positron is an extensible, polyglot
tool for writing code and exploring data in Python, R, and other
languages.
positron-bin 0.3.0-2 AUR
Desktop application that allows you to manage your movie and show
watchlist.(Prebuilt version.Use system-wide electron)
So, it can be installed via pamac’s GUI (make sure you have the AUR enabled in 3rd-party settings, and the base-devel package installed). You can build (install) it via the command line:
pamac build positron-ide-devel-bin
Edit: after checking what RStudio does, I realised you probably are after the IDE, not the movie watchlist app