[react-native] Declare StyleSheet.setStyleAttributePreprocessor

Added the type declaration and copied the documentation for this static function. The type of the `process` parameter should be `(nextProp: unknown) => unknown` but I stuck with `any` to preserve 2.x compatibility.

types/react-native/index.d.ts

@@ -5216,6 +5216,20 @@ export namespace StyleSheet {
     export function flatten(style?: StyleProp<ImageStyle>): ImageStyle;
     export function flatten(style?: StyleProp<ViewStyle>): ViewStyle;
 
+    /**
+     * WARNING: EXPERIMENTAL. Breaking changes will probably happen a lot and will
+     * not be reliably announced. The whole thing might be deleted, who knows? Use
+     * at your own risk.
+     *
+     * Sets a function to use to pre-process a style property value. This is used
+     * internally to process color and transform values. You should not use this
+     * unless you really know what you are doing and have exhausted other options.
+     */
+    export function setStyleAttributePreprocessor(
+        property: string,
+        process: (nextProp: any) => any
+    ): void;
+
     /**
      * This is defined as the width of a thin line on the platform. It can be
      * used as the thickness of a border or division between two elements.

types/react-native/test/index.tsx

@@ -151,6 +151,8 @@ const stylesAlt = StyleSheet.create({
     },
 });
 
+StyleSheet.setStyleAttributePreprocessor('fontFamily', (family: string) => family);
+
 const welcomeFontSize = StyleSheet.flatten(styles.welcome).fontSize;
 
 const viewStyle: StyleProp<ViewStyle> = {