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`)
```
* If no version is matching => we print out a message (like we used to)
* If only one version is matching (because the user provided a specific version `6.17.1` or because the prefix match only one installed version) => the version get uninstalled
* If multiple installed versions are matched: we print out the matching installed versions and ask the user to re-run the command with the correct version to uninstall.
Note: it's not possible to uninstall using `latest` or `latest-v9.x`, ... But that's already the case so nothing has changed here.
Gal Schlezinger