Register extension resource URL scheme as "standard"

> Registering a scheme as standard allows relative and absolute resources to be
  resolved correctly when served.

If, for example, a CSS file does a relative import, the imported path can be
resolved within the extension bundle.

desktop/main/extensionResources.ts

@@ -17,9 +17,7 @@ const NET_ERROR_FAILED = -2;
 export function registerExtensionProtocolHandlers(): void {
   protocol.registerFileProtocol("x-foxglove-extension-rsrc", (request, callback) => {
     // Split the URL into an extension ID and the resource path
-    const urlPathParts = new URL(request.url).pathname.replace(/^\/+/, "").split("/");
-    const extId = urlPathParts[0] ?? "";
-    const rsrcPath = urlPathParts.slice(1).join("/");
+    const { host: extId, pathname: rsrcPath } = new URL(request.url);
 
     const homePath = app.getPath("home");
     const userExtensionRoot = pathJoin(homePath, ".foxglove-studio", "extensions");
@@ -39,3 +37,17 @@ export function registerExtensionProtocolHandlers(): void {
       });
   });
 }
+
+export function registerExtensionProtocolSchemes(): void {
+  protocol.registerSchemesAsPrivileged([
+    {
+      scheme: "x-foxglove-extension-rsrc",
+      privileges: {
+        // The URL scheme adheres to "generic URI syntax", with a host (i.e.,
+        // the extension's ID) and a path. This also allows resolving relative
+        // resources, like a CSS file that uses url("./icon.png")
+        standard: true,
+      },
+    },
+  ]);
+}

desktop/main/index.ts

@@ -13,7 +13,10 @@ import { AppSetting } from "@foxglove/studio-base/src/AppSetting";
 import pkgInfo from "../../package.json";
 import StudioAppUpdater from "./StudioAppUpdater";
 import StudioWindow from "./StudioWindow";
-import { registerExtensionProtocolHandlers } from "./extensionResources";
+import {
+  registerExtensionProtocolHandlers,
+  registerExtensionProtocolSchemes,
+} from "./extensionResources";
 import getDevModeIcon from "./getDevModeIcon";
 import injectFilesToOpen from "./injectFilesToOpen";
 import installChromeExtensions from "./installChromeExtensions";
@@ -194,6 +197,7 @@ function main() {
   ipcMain.handle("getHomePath", () => app.getPath("home"));
 
   // Must be called before app.ready event
+  registerExtensionProtocolSchemes();
   registerRosPackageProtocolSchemes();
 
   ipcMain.handle("updateNativeColorScheme", () => {