Made it easier to test the frontend against an existing core instance. (#23062)

* Made it easier to test the frontend against an existing core instance.

* Ensured that script works regardless of current working dir

* Use consistent quote style

* Also allow using variables in hassUrl override

* Improved the default behavior of the script

* more consistent variable naming

* don't install a global dependency

* documented caching wierdness where if you switch core endpoints the old one remains in use

* Simplified some code

* improved documentation

---------

Co-authored-by: Petar Petrov <MindFreeze@users.noreply.github.com>

build-scripts/bundle.cjs

@@ -53,6 +53,11 @@ module.exports.definedVars = ({ isProdBuild, latestBuild, defineOverlay }) => ({
   __SUPERVISOR__: false,
   __BACKWARDS_COMPAT__: false,
   __STATIC_PATH__: "/static/",
+  __HASS_URL__: `\`${
+    "HASS_URL" in process.env
+      ? process.env["HASS_URL"]
+      : "${location.protocol}//${location.host}"
+  }\``,
   "process.env.NODE_ENV": JSON.stringify(
     isProdBuild ? "production" : "development"
   ),