split the package into nu-themes and nu-hooks (#700)

related to 
- nushell/nupm#50

## description
`nu-scripts` was not really a package but rather a set of two module
packages.
this feature has been removed from Nupm in
nushell/nupm#50 and this PR fixes the
`nu-scripts` packages.

> **Note**
> the files in `nu-hooks/` have been moved to `nu-hooks/nu-hooks/` to
fit into the current model of Nupm

this PR does not change anything from the user point of view, apart from
the install that goes from
```nushell
nupm install --path --force .
```
to
```nushell
for pkg in (ls **/package.nuon | get name | path dirname) {
    nupm install --force --path $pkg
}
```

package.nuon → nu-hooks/package.nuon

@@ -1,9 +1,8 @@
 {
-    name: "nu-scripts"
-    description: "A place to share Nushell scripts with each other"
+    name: "nu-hooks"
+    description: "Officially-supported hooks for Nushell"
     documentation: "https://github.com/nushell/nu_scripts/blob/main/README.md"
     license: "https://github.com/nushell/nu_scripts/blob/main/LICENSE"
     version: 0.1.0
     type: "module"
-    modules: ["./themes/nu-themes/", "./nu-hooks/"]
 }

themes/package.nuon

@@ -0,0 +1,8 @@
+{
+    name: "nu-themes"
+    description: "Officially-supported themes for Nushell"
+    documentation: "https://github.com/nushell/nu_scripts/blob/main/README.md"
+    license: "https://github.com/nushell/nu_scripts/blob/main/LICENSE"
+    version: 0.1.0
+    type: "module"
+}