Support keyword arguments in named scopes

[milk1000cc]

This PR supports keyword arguments in named scopes.

In the current master version, an ArgumentError is raised.
Ruby version is 3.0.2.

class UsersIndex < Chewy::Index
  def self.by_name(name, options:)
  end
end

UsersIndex.limit(10).by_name('Martin', options: 'blah blah blah')
# => wrong number of arguments (given 2, expected 1) (ArgumentError)

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• Commit message starts with [Fix #issue-number] (if the related issue exists).
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• Squashed related commits together.
• Added tests.
• Added an entry to the changelog if the new code introduces user-observable changes. See changelog entry format for details.

[rabotyaga]

Hey @milk1000cc 👋 !
Thank you very much!

[rabotyaga]

@milk1000cc JFYI: these changes broke ruby 2.6, we had to use another approach to make it compatible - #836