Fix logger LoadError

The logger default gem became bundled in Ruby v4.0 [1] and so it needs to be
added as a dependency in the gemspec. This was somehow causing spec failures
due to the LoadError being swallowed in
`YARD::Templates::Helpers::MarkupHelper#load_markup_provider`.

I've also had to add it to the Gemfile, because the specs don't seem to use the
gemspec.

[1]: https://rubyreferences.github.io/rubychanges/4.0.html#default-gems-that-became-bundled

Author: floehopper

Gemfile

@@ -4,6 +4,7 @@ source 'https://rubygems.org'
 group :development do
   if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('4.0.0')
     gem 'irb'
+    gem 'logger'
   end
   gem 'rspec', '>= 3.11.0'
   gem 'rake'

yard.gemspec

@@ -22,5 +22,6 @@ Gem::Specification.new do |s|
   s.metadata['yard.run'] = 'yri'
   if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('4.0.0')
     s.add_dependency 'irb'
+    s.add_dependency 'logger'
   end
 end