docs: correct the externalsForTarget contract and document installedPackagesForTarget

externalsForTarget is synchronous; the example showed async, which the
build consumes incorrectly and TypeScript rejects. Adds a section for the
new diagnostics-only installedPackagesForTarget hook.

Author: matt-aitken

docs/config/extensions/custom.mdx

@@ -80,7 +80,7 @@ export default defineConfig({
     extensions: [
       {
         name: "my-extension",
-        externalsForTarget: async (target) => {
+        externalsForTarget: (target) => {
           return ["my-dependency"];
         },
       },
@@ -89,6 +89,19 @@ export default defineConfig({
 });
 ```
 
+### installedPackagesForTarget
+
+This tells build diagnostics which packages your extension installs into the deployed image for a given target, so warnings (like the one for packages loaded via `createRequire()`) don't fire for packages that will actually be available at runtime. The bundler ignores this hook, so declaring it never changes the build output. Return an empty array to declare that your extension installs no packages, which keeps those diagnostics active in `dev` for projects using your extension.
+
+```ts
+{
+  name: "my-extension",
+  installedPackagesForTarget: (target) => {
+    return target === "deploy" ? ["my-dependency"] : [];
+  },
+}
+```
+
 ### onBuildStart
 
 This hook runs before the build starts. It receives the `BuildContext` object as an argument.