Update header-gen package.json to fix import issue (#8)

* Update header-gen package.json to fix import issue

* Move jest config to json file to handle es module

Author: microbit-robert

header-gen/jest.config.js

@@ -0,0 +1,5 @@
+{
+  "preset": "ts-jest",
+  "testEnvironment": "node",
+  "testMatch": ["**/?(*.)+(spec|test).[tj]s?(x)"]
+}

header-gen/jest.config.json

@@ -4,12 +4,18 @@
   "description": "A simple TS script to generate a header blob for the ML runner",
   "author": "Micro:bit Educational Foundation <[email protected]>",
   "license": "MIT",
+  "type": "module",
   "module": "./built/main.js",
   "typings": "./built/main.d.ts",
   "files": [
     "./built/*.js",
     "./built/*.d.ts"
   ],
+  "exports": {
+    ".": {
+      "import": "./built/main.js"
+    }
+  },
   "scripts": {
     "start": "ts-node --project tsconfig.cli.json src/cli.ts",
     "test": "jest",