Adds `fnm exec` to run a shell executable with the current node version (or a custom one provided):
```
fnm exec -- node -v # will print the current Node version in fnm
fnm exec --using 12 -- node -v # will print the version of the latest node 12 installed
fnm exec --using-file -- node -v # will print the version of the current directory's node version (based on `.nvmrc` or `.node-version`)
```
Seems that Lwt 5 throws a weird error when using `exit(1)` inside a
promise. So instead of exiting in the command, making all executable
modules a function that returns a `result((), status_code)` is a better fit.
Relates to: https://github.com/ocsigen/lwt/issues/758
* Use variants instead of a boolean
* Add infrastructure for multishell and aliases: Aliases are required because I want to have a default node version on startup
* create alias command
* fmt
* Added aliases (Fixes#29) and opt-in multishell support (Fixes#19)
* Better docs
* update snapshot
* Some/Fail => Some/None
* add lint-staged
* use refmt and all of prettier are grouped
* System.readdir => Fs.readdir (now uses Lwt)
* use lstat for file_exists: check if symlink exists instead of actual linked file. also, initialize the Random seed on Env
* Remove fish set options that were added in trial and error
* Bootstrap script
* add bootstrap documentation
Corentin Leruth