Run CI workflow on Windows and macOS (rojo-rbx#825)

This PR adds macOS and Windows jobs to the CI workflow. This allows us
to see when changes break functionality on any supported platform, which
is particularly important for changes that involve the file system or
file watcher.

.github/workflows/ci.yml

@@ -12,11 +12,12 @@ on:
 jobs:
   build:
     name: Build and Test
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
 
     strategy:
       matrix:
         rust_version: [stable, 1.70.0]
+        os: [ubuntu-latest, windows-latest, macos-latest]
 
     steps:
     - uses: actions/checkout@v3