REPL support

[athas]

Some language modes seem to have built-in REPL support. Maybe worth considering. futhark repl shouldn't be difficult to wrangle.

[haoranpb]

Find something in HLS haskell/haskell-language-server#477, but it sounds complicated.

It shouldn't be hard to implement something that creates a terminal and calls futhark repl on launch.

[athas]

It certainly doesn't need to be as complicated as for HLS (e.g. no reason to run the REPL as part as the server). Maybe running futhark repl in a terminal is fine.