fix: 🐛 Declare GenerateManualPlugin as plugin product (#456)

`ArgumentParser` 1.1.3 includes a SwiftPM plugin for generating man
pages. However, this plugin is not visible to packages that declare
`ArgumentParser` as a package dependency.

Defining a product of type `plugin` in the package manifest will solve
that. Running `swift package plugin --list` for a package, that
declares `ArgumentParser` as a package dependency, will then list the
`GenerateManualPlugin`

```
‘experimental-generate-manual’ (plugin ‘GenerateManualPlugin’ in package ‘swift-argument-parser’)
```

Closes: #455

Package@swift-5.6.swift

@@ -18,6 +18,9 @@ var package = Package(
         .library(
             name: "ArgumentParser",
             targets: ["ArgumentParser"]),
+        .plugin(
+            name: "GenerateManualPlugin",
+            targets: ["GenerateManualPlugin"]),
     ],
     dependencies: [],
     targets: [