Run tests on more Ruby versions and platforms

We still exclude truffleruby and jruby, but we should be testing on
Windows now and against Ruby head. This is lifted from Ruby's base64
gem's GitHub workflow and depends on Ruby's own version code which
should give us better coverage.

- https://github.com/ruby/base64/blob/master/.github/workflows/test.yml
- https://github.com/ruby/actions/blob/master/.github/workflows/ruby_versions.yml

.github/workflows/main.yml

@@ -1,4 +1,4 @@
-name: Berns
+name: Test Suite
 
 concurrency:
   cancel-in-progress: true
@@ -7,22 +7,33 @@ concurrency:
 on: [push, pull_request]
 
 jobs:
+  ruby-versions:
+    uses: ruby/actions/.github/workflows/ruby_versions.yml@master
+    with:
+      min_version: 2.7
+
   test:
+    needs: ruby-versions
+    name: Test (${{ matrix.ruby }} / ${{ matrix.os }})
     strategy:
-      fail-fast: false
       matrix:
-        os: [ubuntu-latest, macos-latest]
-        ruby-version: [3.2, 3.1, '3.0', 2.7]
+        ruby: ${{ fromJson(needs.ruby-versions.outputs.versions) }}
+        os: [ubuntu-latest, macos-latest, windows-latest]
+        exclude:
+          - { ruby: jruby }
+          - { ruby: jruby-head }
+          - { ruby: truffleruby }
+          - { ruby: truffleruby-head }
 
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v3
 
-    - name: Set up Ruby ${{ matrix.ruby-version }}
+    - name: Set up Ruby ${{ matrix.ruby }}
       uses: ruby/setup-ruby@v1
       with:
         bundler-cache: true
-        ruby-version: ${{ matrix.ruby-version }}
+        ruby-version: ${{ matrix.ruby }}
 
     - name: Run tests & lint
       run: bundle exec rake suite

README.md

@@ -25,6 +25,10 @@ Or install it yourself as:
 gem install berns
 ```
 
+*Note:* Berns is only tested on Ruby's MRI/CRuby interpreter version 2.7 and
+greater. Berns version 3.x and below support Ruby 2.5. Contributions are welcome
+to get JRuby or TruffleRuby working.
+
 ## Usage
 
 Note that all return string values will be UTF-8 encoded.