Add types using tsc

.gitignore

@@ -4,3 +4,5 @@ __pycache__
 dist
 build
 *.pyc
+
+node_modules/

Makefile

@@ -1,15 +1,21 @@
-index.js src/globalwarmingpotentials/__init__.py: scripts/generate_modules.py globalwarmingpotentials.csv venv
+all: src/globalwarmingpotentials/__init__.py index.js index.d.ts
+
+src/globalwarmingpotentials/__init__.py index.js: scripts/generate_modules.py globalwarmingpotentials.csv venv
 	@./venv/bin/python $<
 	@./venv/bin/black src/globalwarmingpotentials/*.py
 
+index.d.ts: index.js package.json
+	@npm install
+	@npm run build
+
 venv: scripts/requirements.txt
 	[ -d ./venv ] || python3 -m venv venv
 	./venv/bin/pip install --upgrade pip
 	./venv/bin/pip install -Ur $<
 	touch venv
 
 clean-generated-files:
-	rm -rf index.js py/globalwarmingpotentials/__init__.py
+	rm -rf index.js index.d.ts py/globalwarmingpotentials/__init__.py
 
 clean-venv:
 	rm -rf venv