All Minetest games consist purely of Lua, so of course there is. I don't know if there's a way to "sideload" games and mods into the browser storage but installing them from ContentDB works.

Seems like one might be able to use emscripten_wget or something. Emscripten can expose a unix-like filesystem

The official mod API is pure lua:

(as an example:)

https://github.com/tenplus1/minetest_game/blob/master/mods/f...

That code would run server side, but there is some recent work for client-side scripts as well.