`spago run --watch` recompiles but doesn't rerun

[f-f]

It looks like this happens because Spago fires up a Nodejs process that might not terminate. When reloading the watcher the old process is left running and a new one is started.

We should instead keep track of the old ProcessHandle (see here for how to make one) and kill that in the watcher

[wclr]

Yes, it is quite strange, watch but not re-run).

[f-f]

We don't support watching in Spago anymore, see https://github.com/purescript/spago/issues/1029