Add support for 'in:' kwarg in Time.new and Time.now

[mcfadden]

This adds support for Ruby 3.1's in: keyword argument.

Example usage:

Timecop.freeze(Time.new(2024, 9, 20, 1, 2, 30)) do
  puts Time.new # 2024-09-20 01:02:30 -0400
  puts Time.new(in: "+04:00") # 2024-09-20 09:02:30 +0400
  puts Time.now(in: "+03:00") # 2024-09-20 08:02:30 +0300
  puts Time.new(in: "+02:00") # 2024-09-20 07:02:30 +0200
end

Fixes #431

[joshuacronemeyer]

@mcfadden thanks for this! Can you add some usages of this in the unit tests so we can make sure there are assertions that ensure this parameter is handled? Also I noticed the ruby docs use this for the Time.at method as well so can you throw that in as well? Thank you.