You can use the USERNAME environment variable, which seems to be defined by default on Windows, macOS and Linux:
var username
if OS.has_environment("USERNAME"):
username = OS.get_environment("USERNAME")
else:
username = "Player"
The snippet above falls back to Player if the environment variable doesn’t exist for some reason.
Keep in mind this may not be a good solution for multiplayer games, as many people use their real name as their computer user name and would rather stay anonymous.
‘USERNAME’ is not standard (just windows), ‘USER’ is (Linux, MacOS, Android, iOS, …)