release blog: generate release post

GitHub: groongaGH-83

In this PR, we implement a Rake task for generating release
announces in blog without contents.
At the following PRs, we will implement contents step by step.

Generate release announces

```console
$ VERSION=14.1.1 rake release:blog:generate
$ tree . | grep 2024-12-04-groonga-14.1.1.md
  │      └── 2024-12-04-groonga-14.1.1.md
  │   │  └── 2024-12-04-groonga-14.1.1.md
```

Rakefile

@@ -17,5 +17,11 @@
 
 require_relative "release_task"
 
-release_task = ReleaseTask.new("groonga")
+def env_var(name, default=nil)
+  value = ENV[name] || default
+  raise "${#{name}} is missing" if value.nil?
+  value
+end
+
+release_task = ReleaseTask.new("groonga", env_var("VERSION"), __dir__)
 release_task.define

release_task.rb

@@ -18,8 +18,10 @@
 class ReleaseTask
   include Rake::DSL
 
-  def initialize(package)
+  def initialize(package, version, jekyll_path)
     @package = package
+    @version = version
+    @jekyll_path = jekyll_path
   end
 
   def define
@@ -33,9 +35,27 @@ def define_generate_blog_task
       namespace :blog do
         desc "Generate release announce posts from a release note"
         task :generate do
-          puts "TODO: Generate release announce posts for #{@package}"
+          generate_blog_posts
         end
       end
     end
   end
+
+  def post_filename
+    "#{Time.now.strftime("%F")}-#{@package}-#{@version}.md"
+  end
+
+  def post_content(locale)
+    # TODO: We will write blog post contents here.
+    # After writing contents, we will remove this TODO comment.
+    "#{locale}, #{@package}, #{@version}"
+  end
+
+  def generate_blog_posts
+    ["ja", "en"].each do |locale|
+      File.open("#{@jekyll_path}/#{locale}/_posts/#{post_filename}", "w") do |post|
+        post.write(post_content(locale))
+      end
+    end
+  end
 end