Fix error loading ifctester webapp (infinite "initialize environment...")

Browser was stuck trying to initialize the environment, because of the error below:
[worker] Received message: {type: 'init', payload: {…}, id: '9dLz/0eBQDa0xTQORXfdUg/0'} :5000/pyodide/pyodide.mjs:1  Failed to load module script: Expected a JavaScript-or-Wasm module script but the server responded with a MIME type of "text/plain". Strict MIME type checking is enforced for module scripts per HTML spec.

src/ifctester/webapp/serve.py

@@ -18,8 +18,13 @@
 
 import os
 import argparse
+import mimetypes
 from flask import Flask, send_from_directory, send_file
 
+# Browsers expecting this specific MIME type for .mjs files,
+# otherwise they fail to load.
+mimetypes.add_type("text/javascript", ".mjs")
+
 app = Flask(__name__)