feat: Add support for validating multiple attributes at once

[matsales28]

This commit adds support for asserting against multiple attributes in a
single matcher expression. Instead of writing one expectation per
attribute, you can pass the attributes as positional arguments:

class Robot
 include ActiveModel::Model
 attr_accessor :arms, :legs

 validates_presence_of :arms, :legs
end

RSpec.describe Robot, type: :model do
 it { expect(subject).to validate_presence_of(:arms, :legs) }
end

This is implemented by a new Shoulda::Matchers::MatcherCollection
class that wraps individual matchers and produces a combined
description and per-attribute failure reasons in a single message.

The following matchers now support multiple attributes:

• validate_presence_of
• validate_absence_of
• validate_acceptance_of
• validate_confirmation_of
• validate_length_of
• validate_numericality_of
• validate_inclusion_of
• validate_exclusion_of
• validate_comparison_of
• have_readonly_attribute
• have_one_attached

Qualifiers chained on the matcher are applied to every wrapped
attribute uniformly. For example:

expect(subject).to validate_presence_of(:arms, :legs).allow_nil

asserts that both :arms and :legs allow nil. If any attribute
fails the expectation, the failure message lists every failed
attribute and the specific reason each one failed.