Add CI.

.github/dependabot.yml

@@ -0,0 +1,8 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions" # Necessary to update action hashs	
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    # Allow up to 3 opened pull requests for github-actions versions
+    open-pull-requests-limit: 3

.github/workflows/ci.yml

@@ -0,0 +1,27 @@
+# Copyright (C) 2024 Toitware ApS.
+#
+# Use of this source code is governed by a BSD0-style license that can be
+# found in the LICENSE_BSD0 file.
+
+name: Build
+
+on:
+  push
+
+jobs:
+  build:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Toit build environment
+        uses: ./toit/actions/setup-build
+
+      - name: Build
+        shell: bash
+        run: |
+          make