https://github.com/ruby/pathname/commit/b020cbde46

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
```

When I want to create a tmpdir I often want to manipulate it as a pathname. By introducing Pathname.mktmpdir I can get this behavior. 

Currently I must:

```ruby
Dir.mktmpdir do |dir|
  dir = Pathname(dir)
  # ... code
end
```

I would like to be able to instead:

```ruby
Pathname.mktmpdir do |dir|
  # ... code
end
```

delete_prefix with a string is easier to read than a regular expression
also it should be faster. It is available since ruby 2.5 and the gem requires
ruby 2.7.

https://github.com/ruby/pathname/commit/0070f43f19

rmtree is already requiring fileutils, but mkpath apparently relies on
autoload of FileUtils. Switch to require for both methods

https://github.com/ruby/pathname/commit/07ad1fb41a

https://github.com/ruby/pathname/commit/f3d23679b0

https://github.com/ruby/pathname/commit/2b0b1a82ee

https://github.com/ruby/pathname/commit/3cb5ed2576

Should not be `Pathname::FileUtils`.

https://github.com/ruby/pathname/commit/d1eb366e73