Add Steep for type checking

Add `steep` to Gemfile, add `steep` rake task, and add `typecheck`
GitHub workflow.

Author: sferik

.github/workflows/typecheck.yml

@@ -0,0 +1,18 @@
+name: typecheck
+
+on: [push, pull_request]
+
+jobs:
+  steep:
+    runs-on: ubuntu-latest
+    env:
+      BUNDLE_GEMFILE: Gemfile.steep
+    steps:
+    - uses: actions/checkout@v6
+    - name: Set up Ruby
+      uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: "ruby"
+        bundler-cache: true
+    - name: Type check
+      run: bundle exec rake steep

Gemfile.steep

@@ -0,0 +1,5 @@
+source "https://rubygems.org"
+
+gem "rake"
+gem "rbs", "~> 3.9"
+gem "steep"

Rakefile

@@ -8,6 +8,11 @@ Rake::TestTask.new(:test) do |t|
   t.test_files = FileList["test/**/test_*.rb"]
 end
 
+desc "Type check with Steep"
+task :steep do
+  sh "steep check"
+end
+
 namespace :rbs do
   Rake::TestTask.new(:test) do |t|
     t.libs << "test_sig"

Steepfile

@@ -0,0 +1,17 @@
+target :lib do
+  check "lib"
+  signature "sig"
+
+  library "cgi"
+  library "open-uri"
+  library "net-protocol"
+  library "openssl"
+  library "resolv"
+  library "securerandom"
+  library "socket"
+  library "strscan"
+  library "tempfile"
+  library "timeout"
+  library "uri"
+  library "zlib"
+end