Add tsconfig.json to fix VSCode TypeScript errors

Without a tsconfig.json VSCode defaulted to ESM module mode, making
import X = require() invalid. Explicit module: commonjs aligns VSCode
with the typecheck script, which is simplified to just tsc.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: danmactough

package.json

@@ -57,7 +57,7 @@
   },
   "scripts": {
     "lint": "eslint .",
-    "typecheck": "tsc --noEmit --strict test/types.ts --lib es6 --module commonjs",
+    "typecheck": "tsc",
     "pretest": "npm run lint && npm run typecheck",
     "test": "mocha",
     "version": "git changelog ; git add History.md"

tsconfig.json

@@ -0,0 +1,11 @@
+{
+  "compilerOptions": {
+    "noEmit": true,
+    "strict": true,
+    "module": "commonjs",
+    "lib": ["es6"]
+  },
+  "files": [
+    "test/types.ts"
+  ]
+}