Stumbled on this when looking up the game engine in the
splash page of a game demo I was playing.
"Godot game engine"
https://godotengine.org/
Seems to have many of the usual features, a fair bit of
documentation and a public project to maintain it - it can
generate games on multiple platforms, and it is free to make
whatever with, and it appears (given the game demo I was
looking at had a paid edition) you are allowed to sell the
games you make with it - apparently without restriction.
Similar building platforms typically sell for 100 or more on
steam.
Not sure how friendly it is to use, but they have a few demo
projects and such, and it seems to support a few scripting
languages c+ C# GD etc
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