build: make npm run build self-sufficient

The ts-cleaner step in the build script scans dist/ and crashes with
ENOENT if the directory doesn't exist (e.g. on a fresh clone or after
git clean). Previously README told users to 'mkdir -p dist' first,
but Dockerfile and CI didn't necessarily follow that. Prepend a small
node one-liner that mkdir's dist recursively before ts-cleaner runs,
and drop the now-redundant manual step from README.

package.json

@@ -15,7 +15,7 @@
   "type": "commonjs",
   "main": "dist/index.js",
   "scripts": {
-    "build": "ts-cleaner && tsc",
+    "build": "node -e \"require('fs').mkdirSync('dist',{recursive:true})\" && ts-cleaner && tsc",
     "start": "node .",
     "dev": "ts-node src/index.ts",
     "prod": "ts-node src/utils/Prod-commands.ts",