Add a cut_release rake task

The task is mostly copied from RuboCop. I somehow couldn't get this line to work:

  Rake::Task['update_cops_documentation'].invoke

which is why we shell out instead:

  `bundle exec rake generate_cops_documentation`

.rubocop.yml

@@ -1,3 +1,5 @@
+inherit_from: .rubocop_todo.yml
+
 require:
   - rubocop-performance
   - rubocop-rake

.rubocop_todo.yml

@@ -0,0 +1,11 @@
+# This configuration was generated by
+# `rubocop --auto-gen-config --no-offense-counts --no-auto-gen-timestamp`
+# using RuboCop version 1.36.0.
+# The point is for the user to remove these configuration records
+# one by one as the offenses are removed from the code base.
+# Note that changes in the inspected code, or installation of new
+# versions of RuboCop, may require this file to be generated again.
+
+Rake/MethodDefinitionInTask:
+  Exclude:
+    - 'tasks/cut_release.rake'

Gemfile

@@ -4,6 +4,7 @@ source 'https://rubygems.org'
 
 gemspec
 
+gem 'bump'
 gem 'rack'
 gem 'rake'
 gem 'rspec', '~> 3.11'

tasks/cut_release.rake

@@ -0,0 +1,57 @@
+# frozen_string_literal: true
+
+require 'bump'
+
+namespace :cut_release do
+  def update_file(path)
+    content = File.read(path)
+    File.write(path, yield(content))
+  end
+
+  %w[major minor patch pre].each do |release_type|
+    desc "Cut a new #{release_type} release and update documents."
+    task release_type do
+      run(release_type)
+    end
+  end
+
+  def version_sans_patch(version)
+    version.split('.').take(2).join('.')
+  end
+
+  # Replace `<<next>>` (and variations) with version being cut.
+  def update_cop_versions(version)
+    update_file('config/default.yml') do |default|
+      default.gsub(/['"]?<<\s*next\s*>>['"]?/i,
+                   "'#{version_sans_patch(version)}'")
+    end
+    RuboCop::ConfigLoader.default_configuration = nil # invalidate loaded conf
+  end
+
+  def update_docs(version)
+    update_file('docs/antora.yml') do |antora_metadata|
+      antora_metadata.sub('version: master',
+                          "version: '#{version_sans_patch(version)}'")
+    end
+  end
+
+  def add_header_to_changelog(version)
+    update_file('CHANGELOG.md') do |changelog|
+      changelog.sub("## Master (Unreleased)\n\n",
+                    '\0' "## #{version} (#{Time.now.strftime('%F')})\n\n")
+    end
+  end
+
+  def run(release_type)
+    old_version = Bump::Bump.current
+    Bump::Bump.run(release_type, commit: false, bundle: false, tag: false)
+    new_version = Bump::Bump.current
+
+    update_cop_versions(new_version)
+    `bundle exec rake generate_cops_documentation`
+    update_docs(new_version)
+    add_header_to_changelog(new_version)
+
+    puts "Changed version from #{old_version} to #{new_version}."
+  end
+end