Pass options as keyword arguments to translation procs

[casperisfine]

So a pattern that's heavily used in our translation data, is to use keyword arguments in translations procs rather than an option hash, e.g:

lambda do |_key, number:, **|
  if number == 1
    'er'
  else
    'e'
end

Instead of:

lambda do |_key, options|
  if options.fetch(:number) == 1
    'er'
  else
    'e'
end

It makes for slightly more concise code, and gives better error messages if a key is missing.

However since the signature I18n call is call(String, Hash) this causes warnings on 2.7 and will break on 2.8/3.0.

gem/ruby/2.7.1/gems/i18n-1.8.2/lib/i18n/backend/base.rb:149: warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call
config/locales/rules/de.rb:8: warning: The called method `call' is defined here

I can off course rewrite these callbacks, but since I believe using keyword args is often nicer, I wanted to check if there was interest in updating the signature.

AFAIK this is backward compatible, as call(String, **Hash) will automatically pass the keyword args as a positional hash if the proc doesn't accept keyword args:

l = lambda do |_key, options|
  if options.fetch(:number) == 1
    'er'
  else
    'e'
  end
end
hash = { number: 1}
p l.call('foo', **hash) # No warnings

[radar]

Changes look good to me. Tests are happy with them too. Thank you for taking the time to submit this PR.

[casperisfine]

Wow, thanks a lot for the quick merge!