Allow multiple attributes to be defined in single cattr/iattr call

[bnlucas]

Adds the ability to define multiple attributes at once.

class Config
  include Cattri

  cattr :a, :b.                          # defines self.a/a=, self.b/b= defaulting to nil.
  cattr_reader :c, :d, default: false    # defines self.c, self.d readers, defaulting to false.

  cattr :e, :f do |val|                  # raises Cattri::AmbiguousBlockError, blocks not allow
    val                                  # for multiple attribute definitions.
  end

class Config
  include Cattri

  iattr :a, :b                           # defines new.a/a=, new.b/b= defaulting to nil.
  iattr_reader :c, :d, default: false    # defines new.a, new.b readers, defaulting to false.
  iattr_writer :e, :f                    # defines new.a, new.b writers, setters are not allowed.

  iattr :e, :f do |val|                  # raises Cattri::AmbiguousBlockError, blocks not allow
    val                                  # for multiple attribute definitions.
  end
end

Due to multiple attribute definitions not allowing an ambiguous setter block to provided, this also introduces class_attribute_setter/cattr_setter and instance_attribute_setter/iattr_setter

class Config
  include Cattri

  cattr :a, :b                          # defines def (a|b)=(v); @(a|b) = v; end

  cattr_setter :a do |val|              # applies a setter to the :a variable, changing it
    val.to_s.downcase.to_sym            # to def a=(v); @a = v.to_s.downcase.to_sym; end
  end

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (df2777b) to head (499cf43).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main        #2   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines          278       311   +33     
  Branches        27        34    +7     
=========================================
+ Hits           278       311   +33     

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