Add build scripts.

.github/workflows/build.yml

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+name: Build
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+
+      - name: 'Checkout code'
+        uses: actions/checkout@v3
+
+      - name: 'Tests'
+        uses: subosito/flutter-action@v2
+        with:
+          flutter-version: '3.x'
+          channel: 'stable'
+      - run: flutter pub get
+      - run: make check

Makefile

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+default: check
+
+.PHONY: tasks
+tasks: ## Print available tasks
+	@printf "\nUsage: make [target]\n\n"
+	@grep -E '^[a-z][^:]+:.*?## .*$$' $(MAKEFILE_LIST) | sort | awk 'BEGIN {FS = ":.*?## "}; {printf "\033[36m%-20s\033[0m %s\n", $$1, $$2}'
+
+.PHONY: format
+format: ## Format code
+	dart format lib --line-length 100 --set-exit-if-changed
+
+.PHONY: test
+test: ## Run tests
+	flutter test
+
+.PHONY: check
+check: format test ## Check formatting and run tests