cmd/viewer: add support for map-like container types

This PR modifies viewTypeForContainerType to use the last type parameter of a container type
as the value type, enabling the implementation of map-like container types where the second-to-last
(usually first) type parameter serves as the key type.

It also adds a MapContainer type to test the code generation.

Updates #12736

Signed-off-by: Nick Khyl <nickk@tailscale.com>

cmd/viewer/viewer.go

@@ -448,7 +448,7 @@ func viewTypeForContainerType(typ types.Type) (*types.Named, *types.Func) {
 	}
 	// ...and add the element view type.
 	// For that, we need to first determine the named elem type...
-	elemType, ok := baseType(containerType.TypeArgs().At(0)).(*types.Named)
+	elemType, ok := baseType(containerType.TypeArgs().At(containerType.TypeArgs().Len() - 1)).(*types.Named)
 	if !ok {
 		return nil, nil
 	}