Support Thor::CoreExt::HashWithIndifferentAccess#slice method

[shuuuuun]

resolve #804

This PR supports Thor::CoreExt::HashWithIndifferentAccess#slice.

hash = Thor::CoreExt::HashWithIndifferentAccess.new 'foo' => 'bar', 'baz' => "bee"
hash.slice('foo')
# => {"foo"=>"bar"}
hash.slice(:foo)
# => {"foo"=>"bar"}

Thank you. 🌈

[shuuuuun]

Oh, this gem needs to support from Ruby 2.0...

[shuuuuun]

Hash#slice was added in Ruby 2.5, so earlier versions cannot support this.
https://docs.ruby-lang.org/en/3.2/NEWS/NEWS-2_5_0.html#:~:text=Hash%23slice%20%5BFeature%20%238499%5D
https://bugs.ruby-lang.org/issues/8499

[rafaelfranca]

Can you rebase this PR now?

[shuuuuun]

I have rebased the PR!
It looks like the CI fail is the same as the one currently occurring in the main branch.
https://github.com/rails/thor/actions/runs/4953317981

I'll rebase again once the issue is resolved in the main branch.

[rafaelfranca]

Fixed