add a starter toolkit to run the tests (#47)

this PR adds a very simple `toolkit.nu` to run the tests of
`nu-git-manager` without worrying about `use nupm` and the exact
commands to run:
```nushell
use toolkit.nu  # i use a hook for that
toolkit test
```

toolkit.nu

@@ -0,0 +1,15 @@
+# run the tests of the `nu-git-manager` package
+#
+# > **Important**  
+# > the `toolkit test` command requires [Nupm](https://github.com/nushell/nupm) to be installed
+export def "test" [
+    --verbose # show the output of each tests
+] {
+    use nupm
+
+    if $verbose {
+        nupm test --show-stdout
+    } else {
+        nupm test
+    }
+}