Basic CI Testing (gbdev#24)

Just runs the file through rgbasm to make sure the syntax is correct and there aren't any errors.

.github/workflows/testing.yml

@@ -0,0 +1,33 @@
+name: Testing
+on:
+  - push
+  - pull_request
+
+jobs:
+  macos-testing:
+    runs-on: macos-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Install RGBDS
+        run: |
+          brew install rgbds
+      - name: Run rgbasm on hardware.inc
+        run: |
+          rgbasm hardware.inc
+  
+  windows-testing:
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Install RGBDS
+        run: |
+          # Find the URL of the latest Win64 release with the GitHub API
+          $githubLatestReleasesAPI = "https://api.github.com/repos/gbdev/rgbds/releases/latest"
+          $githubReleaseURLs = ((Invoke-WebRequest $githubLatestReleasesAPI) | ConvertFrom-Json).assets.browser_download_url
+          $githubWinReleaseURL = ($githubReleaseURLs | Select-String "win64").ToString()
+          # Download latest release
+          Invoke-WebRequest $githubWinReleaseURL -OutFile rgbds.zip
+          Expand-Archive rgbds.zip
+      - name: Run rgbasm on hardware.inc
+        run: |
+          rgbds/rgbasm.exe hardware.inc