Allow method chaining with Pathname#mkpath

Currently in my code when I want to create a pathname object and create a path at the same time I must use tap

```
path = Pathname.new("/tmp/new").tap(&:mkpath)
```

I think it would be cleaner to be able to chain on the results of these methods instead:

```
path = Pathname.new("/tmp/new").mkpath
```

ext/pathname/lib/pathname.rb

@@ -588,7 +588,7 @@ class Pathname    # * FileUtils *
   def mkpath(mode: nil)
     require 'fileutils'
     FileUtils.mkpath(@path, mode: mode)
-    nil
+    self
   end
 
   # Recursively deletes a directory, including all directories beneath it.
@@ -599,7 +599,7 @@ class Pathname    # * FileUtils *
     # File::Path provides "mkpath" and "rmtree".
     require 'fileutils'
     FileUtils.rm_rf(@path, noop: noop, verbose: verbose, secure: secure)
-    nil
+    self
   end
 end
 
@@ -619,4 +619,3 @@ class Pathname    # * tmpdir *
     end
   end
 end
-