Add Release GitHub Actions workflow

This commit adds a GitHub Actions workflow that will build and publish a
new version of the gem to RubyGems. The workflow follows the pattern
described in RubyGems' "Trusted Publishing" documentation [1].

The workflow is triggered whenever a new Release is published by anyone
with permissions to do so on the project's GitHub repository. This
should work for pre-release versions, as well.

Releases may be created via the website [2] or by using the GitHub CLI:

```sh
gh release create v0.1.0 --generate-notes
```

Being sure to swap out `v0.1.0` with an appropriate (new or existing)
tag, of course. Note that draft releases and/or editing existing
releases will not re-trigger this workflow.

This commit follows up on my question/comment [3] on #3.

[1] https://guides.rubygems.org/trusted-publishing/releasing-gems/
[2] https://github.com/github-community-projects/graphql-client/releases
[3] #3 (comment)

.github/workflows/release.yml

@@ -0,0 +1,20 @@
+name: Release
+
+on:
+  release:
+    types: [published]
+
+jobs:
+  release:
+    name: Release to RubyGems
+    permissions:
+      contents: write
+      id-token: write
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checout@v4
+      - uses: ruby/setup-ruby@v1
+        with:
+          bundler-cache: true
+          ruby-version: ruby
+      - uses: rubygems/release-gem@v1