Add `Pathname#touch` as a facade for `FileUtils.touch`

Hello,

I am in the process to refactoring some code by replacing usage of File and FileUtils with Pathnames and I was surprised to discover that Pathname#touch does not exist.

Would a pull request for such a method be considered?

Definition of Done

Simple usecase

path = Pathname.new('file.txt')
path.touch

# Would be strictly equivalent to:
FileUtils.touch('file.txt')

With all the optional arguments

path = Pathname.new('file.txt')
time = Time.now - 3600
path.touch(mtime: time, nocreate: true, noop: true, verbose: true)

# Would be strictly equivalent to:
FileUtils.touch('file.txt', mtime: time, nocreate: true, noop: true, verbose: true)

Pathname API のエントリポイントから始めて touch メソッドを追加し、その引数を FileUtils.touch と比較します。Pathname#touch が単純な呼び出しと、一覧にある mtime、nocreate、noop、verbose オプションを同等の動作でサポートすれば完了です。