Fix type of settings if nothing is registered yet

Author: wooorm

index.d.ts

@@ -22,6 +22,9 @@ export type {
 
 export {unified} from './lib/index.js'
 
+// See: <https://github.com/sindresorhus/type-fest/blob/main/source/empty-object.d.ts>
+declare const emptyObjectSymbol: unique symbol
+
 /**
  * Interface of known results from compilers.
  *
@@ -98,5 +101,6 @@ export interface Data {
  * export {} // You may not need this, but it makes sure the file is a module.
  * ```
  */
-// eslint-disable-next-line @typescript-eslint/no-empty-interface
-export interface Settings {}
+export interface Settings {
+  [emptyObjectSymbol]?: never
+}

index.test-d.ts

@@ -464,10 +464,6 @@ declare module './index.js' {
   interface CompileResultMap {
     ReactNode: ReactNode
   }
-
-  interface Data {
-    something?: string | undefined
-  }
 }
 
 // Compile plugin (to a non-node).
@@ -552,6 +548,15 @@ expectType<Processor>(
   })
 )
 
+/**
+ * Register our setting.
+ */
+declare module './index.js' {
+  interface Settings {
+    something?: string | undefined
+  }
+}
+
 expectType<Processor>(
   unified().use({
     plugins: [