There is, but it's one of the few features reserved for the pro version ($10)

https://github.com/nesbox/TIC-80/wiki/external-editor

Yes, you can write code in a separate file and then import it to cartridge with tic-80 console commands[1]

You don't have to do this manually every time, just make a shell script with multiple commands; e.g. this will import code, save and run the cartridge:

    ./tic80 --skip --fs="$PWD" --cmd="load cart.tic & import code main.lua & save & run

The other way is indeed to buy or build yourself a PRO version, it can work with cartridges in plaintext format.

[1] https://github.com/nesbox/TIC-80/wiki/import

One of main differences with Pico-8 is that Tic-80 supports many more programming languages.

You can write games in Lua, Moonscript, Javascript, Ruby, Wren, Fennel, Squirrel, Janet and even Python.

Or use anything that compiles to WASM. I've recently created my own bindings and a template for building Tic-80 games in Nim[1]

[1] https://codeberg.org/janakali/tic80-wasm-nim

Perhaps my favorite fantasy console is one of the first, if not the first to exist: CHIP-8. It was originally an easy way to program video games for the Cosmac VIP hobbyist computer, but it found new life in recent years because writing a CHIP-8 interpreter is a good "babby's first emulator" exercise. It's not really capable of much by today's standards, but it goes to show that the fantasy console phenomenon has a long and deep history.