Promote tmpdir to the default gems

lib/tmpdir.gemspec

@@ -0,0 +1,23 @@
+Gem::Specification.new do |spec|
+  spec.name          = "tmpdir"
+  spec.version       = "0.1.0"
+  spec.authors = ["Yukihiro Matsumoto"]
+  spec.email = ["matz@ruby-lang.org"]
+
+  spec.summary       = %q{Extends the Dir class to manage the OS temporary file path.}
+  spec.description   = %q{Extends the Dir class to manage the OS temporary file path.}
+  spec.homepage      = "https://github.com/ruby/tmpdir"
+  spec.required_ruby_version = Gem::Requirement.new(">= 2.3.0")
+
+  spec.metadata["homepage_uri"] = spec.homepage
+  spec.metadata["source_code_uri"] = spec.homepage
+
+  # Specify which files should be added to the gem when it is released.
+  # The `git ls-files -z` loads the files in the RubyGem that have been added into git.
+  spec.files         = Dir.chdir(File.expand_path('..', __FILE__)) do
+    `git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }
+  end
+  spec.bindir        = "exe"
+  spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
+  spec.require_paths = ["lib"]
+end

tool/sync_default_gems.rb

@@ -57,6 +57,7 @@
 # * https://github.com/ruby/did_you_mean
 # * https://github.com/ruby/weakref
 # * https://github.com/ruby/tempfile
+# * https://github.com/ruby/tmpdir
 #
 
 require 'fileutils'
@@ -112,6 +113,7 @@ $repositories = {
   did_you_mean: "ruby/did_you_mean",
   weakref: "ruby/weakref",
   tempfile: "ruby/tempfile",
+  tmpdir: "ruby/tmpdir",
 }
 
 def sync_default_gems(gem)