Start a nupm package registry (#68)

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## Description
<!-- describe in a few words the changes -->

This adds a registry for nupm packages following a format proposed in
#56.

I'm making a separate PR to allow testing
#56 before merging it.

registry.nuon

@@ -0,0 +1,46 @@
+# Registry is a NUON file that tells nupm where to look for packages
+#
+# Currently, these package types are supported:
+# * git ..... package is an online git repository
+# * local ... package is on a local filesystem
+#
+# The package type defines the columns of the registry record. Each column holds
+# holds a table of packages.
+#
+# Git packages accept the following columns:
+# * name ....... name of the package
+# * version .... version of the package
+# * url ........ Git URL of the package
+# * revision ... Git revision to check out when installing the package (commit
+#                hash, version, branch name, ...)
+# * path ....... Path relative to the repository root where to look for nupm.nuon
+#
+# Local packages accept the following columns:
+# * name ....... name of the package
+# * version .... version of the package
+# * path ....... Path where to look for nupm.nuon. It can be absolute, or
+#                relative. If relative, then relative to the registry file.
+#
+# One registry can contain packages with the same name, but they must be of
+# different version.
+
+{
+    git: [
+        [name version url revision path];
+        [
+            nu-git-manager
+            0.5.0
+            https://github.com/amtoine/nu-git-manager.git
+            0.5.0
+            pkgs/nu-git-manager
+        ]
+
+        [
+            nu-git-manager-sugar
+            0.5.0
+            https://github.com/amtoine/nu-git-manager.git
+            0.5.0
+            pkgs/nu-git-manager-sugar
+        ]
+    ]
+}