fix: change to output as ES modules (#365)

This library is broken since v6.0.1 after PR #356. Users face this stack trace:

```
Error [ERR_REQUIRE_ESM]: Must use import to load ES Module: /Users/philip_mcmahon/code/editions/projects/aws/node_modules/read-pkg-up/index.js
require() of ES modules is not supported.
require() of /Users/philip_mcmahon/code/editions/projects/aws/node_modules/read-pkg-up/index.js from /Users/philip_mcmahon/code/editions/projects/aws/node_modules/@guardian/cdk/lib/constants/library-info.js is an ES module file as it is a .js file whose nearest parent package.json contains "type": "module" which defines all .js files in that package scope as ES modules.
Instead rename index.js to end in .cjs, change the requiring code to use import(), or remove "type": "module" from /Users/philip_mcmahon/code/editions/projects/aws/node_modules/read-pkg-up/package.json.
```

This change updates tsconfig to output as ES modules rather than commonjs. This slightly goes against the recommendations (https://github.com/guardian/recommendations/blob/master/npm-packages.md#compiling), however as the library only gets used in controlled environments, this should be fine.

tsconfig.json

@@ -1,9 +1,10 @@
 {
   "compilerOptions": {
     "target": "ES2020",
-    "module": "commonjs",
+    "module": "ES2020",
     "moduleResolution": "node",
     "declaration": true,
+    "declarationMap": true,
     "esModuleInterop": true,
     "strict": true,
     "noUnusedLocals": true,
@@ -14,7 +15,7 @@
     "inlineSources": true,
     "experimentalDecorators": true,
     "typeRoots": ["./node_modules/@types"],
-    "outDir": "lib",
+    "outDir": "lib"
   },
   "include": ["src/**/*"],
   "exclude": ["node_modules", "test", "src/**/*.test.ts", "src/**/__snapshots__/**", "src/**/__mocks__/**"]