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

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.

require fileutils in both methods using it

File.birthtime has existed since Ruby 2.2, and pathname requires Ruby
>= 2.7.0, so the method will always be there.

Remove check for File.birthtime

use delete_prefix instead of sub in find method

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
```

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
```