Fixes Zero / Zero / Zero / name handling

Prior to this change `Zero / Zero / Zero / name` broke as folllows:

```
scala> Zero / Zero / Zero / name
Zero / Zero / Zero / name
<console>:18: error: inferred type arguments [sbt.Zero.type] do not conform to method /'s type parameter bounds [K <: sbt.SlashSyntax.Key[K]]
       Zero / Zero / Zero / name
                   ^
```

main/src/main/scala/sbt/SlashSyntax.scala

@@ -1,6 +1,7 @@
 package sbt
 
 import sbt.librarymanagement.Configuration
+import sbt.internal.util.AttributeKey
 
 /**
  * SlashSyntax implements the slash syntax to scope keys for build.sbt DSL.
@@ -57,7 +58,12 @@ object SlashSyntax {
   }
 
   /** RichConfiguration wraps a configuration to provide the `/` operator for scoping. */
-  final class RichConfiguration(protected val scope: Scope) extends RichScopeLike
+  final class RichConfiguration(protected val scope: Scope) extends RichScopeLike {
+
+    // This is for handling `Zero / Zero / Zero / name`.
+    def /(taskAxis: ScopeAxis[AttributeKey[_]]): RichScope =
+      new RichScope(scope.copy(task = taskAxis))
+  }
 
   /** Both `Scoped.ScopingSetting` and `Scoped` are parents of `SettingKey`, `TaskKey` and
    * `InputKey`. We'll need both, so this is a convenient type alias. */

sbt/src/sbt-test/project/unified/build.sbt

@@ -10,6 +10,7 @@ lazy val root = (project in file("."))
     Compile / console / scalacOptions += "-Ywarn-numeric-widen",
     projA / Compile / console / scalacOptions += "-feature",
     Zero / Zero / name := "foo",
+    Zero / Zero / Zero / name := "foo",
 
     libraryDependencies += uTest % Test,
     testFrameworks += new TestFramework("utest.runner.Framework"),