types: make SchemaType static setters property accessible in TypeScript

Fix #14879

Author: vkarpov15

test/types/schema.test.ts

@@ -1634,3 +1634,7 @@ function gh14825() {
   type SchemaType = InferSchemaType<typeof schema>;
   expectAssignable<User>({} as SchemaType);
 }
+
+function gh14879() {
+  Schema.Types.String.setters.push((val?: unknown) => typeof val === 'string' ? val.trim() : val);
+}

types/schematypes.d.ts

@@ -216,6 +216,9 @@ declare module 'mongoose' {
     /** Attaches a getter for all instances of this schema type. */
     static get(getter: (value: any) => any): void;
 
+    /** Array containing default setters for all instances of this SchemaType */
+    static setters: Function[];
+
     /** The class that Mongoose uses internally to instantiate this SchemaType's `options` property. */
     OptionsConstructor: SchemaTypeOptions<T>;