fix: add missing printer.preprocess definition (#15123)

* fix: allow preprocess to return a promise

* fix: types

* chore: format

* chore: changelog

* Update changelog_unreleased/api/15123.md

Co-authored-by: fisker Cheung <lionkay@gmail.com>

* apply changes

* Update 15123.md

* Update 15123.md

* Simplify changelog

---------

Co-authored-by: fisker Cheung <lionkay@gmail.com>

changelog_unreleased/api/15123.md

@@ -0,0 +1,10 @@
+#### Add missing type definition for `printer.preprocess` (#15123 by @so1ve)
+
+```diff
+export interface Printer<T = any> {
+  // ...
++ preprocess?:
++   | ((ast: T, options: ParserOptions<T>) => T | Promise<T>)
++   | undefined;
+}
+```

src/index.d.ts

@@ -485,6 +485,9 @@ export interface Printer<T = any> {
         | Doc
         | null)
     | undefined;
+  preprocess?:
+    | ((ast: T, options: ParserOptions<T>) => T | Promise<T>)
+    | undefined;
   insertPragma?: (text: string) => string;
   /**
    * @returns `null` if you want to remove this node