ci: add simple github building and testing workflow

.github/workflows/main.yml

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+name: Ruby
+
+on:
+  push:
+    branches: 
+      - main
+
+  pull_request:
+    branches: 
+      - main
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    name: Ruby ${{ matrix.ruby }}
+    strategy:
+      matrix:
+        ruby:
+          - '2.7.8'
+          - '3.3.8'
+
+    steps:
+    - uses: actions/checkout@v4
+    - name: Set up Ruby
+      uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: ${{ matrix.ruby }}
+        bundler-cache: true
+    - name: bundle
+      run: bundle install
+    - name: compile
+      run: | 
+        cd ext/inchi-gem/
+        ruby extconf.rb && make
+    - name: test
+      run: bundle exec rake

.gitignore

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+*.gem
+.bundle
+Gemfile.lock
+pkg/*
+
+# ignore build artifacts
+ext/inchi-gem/Makefile
+ext/inchi-gem/inchi/
+ext/inchi-gem/inchi.o
+ext/inchi-gem/inchi_wrap.*
+
+
+.tool-versions

Gemfile

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+# frozen_string_literal: true
+
+source "https://rubygems.org"
+
+gemspec
+
+gem "rake", "~> 13.0"
+
+gem "test-unit", "~> 3.0"
+
+gem "rubocop", "~> 1.21"