types(schema): add missing search index types

Support for MongoDB Atlas search indexes was added in [#14251](#14251)
but the types for `Schema.searchIndex` and the `autoSearchIndex` option
were missed.

Add these types so that the typescript definitions match the
capabilities of the JavaScript version of `mongoose`.

types/index.d.ts

@@ -274,6 +274,13 @@ declare module 'mongoose' {
     /** Defines an index (most likely compound) for this schema. */
     index(fields: IndexDefinition, options?: IndexOptions): this;
 
+    /**
+     * Define a search index for this schema.
+     *
+     * @remarks Search indexes are only supported when used against a 7.0+ Mongo Atlas cluster.
+     */
+    searchIndex(description: mongodb.SearchIndexDescription): Promise<string>;
+
     /**
      * Returns a list of indexes that this schema declares, via `schema.index()`
      * or by `index: true` in a path's options.

types/schemaoptions.d.ts

@@ -24,6 +24,11 @@ declare module 'mongoose' {
      * automatic index builds, you can set autoIndex to false.
      */
     autoIndex?: boolean;
+    /**
+     * Similar to autoIndex, except for automatically creates any Atlas search indexes defined in your
+     * schema. Unlike autoIndex, this option defaults to false.
+     */
+    autoSearchIndex?: boolean;
     /**
      * If set to `true`, Mongoose will call Model.createCollection() to create the underlying collection
      * in MongoDB if autoCreate is set to true. Calling createCollection() sets the collection's default