Add flowTypes for codegen LIBRARY_GENERATORS (facebook#51987)

Summary:
Pull Request resolved: facebook#51987

In this diff I'm adding flowTypes for codegen LIBRARY_GENERATORS

changelog: [internal] internal

Reviewed By: huntie

Differential Revision: D76470808

fbshipit-source-id: 8e2bddeda1f9175fd25fee04f8fdd3cb7c7faa49

Author: mdvacca

packages/react-native-codegen/src/generators/RNCodegen.js

@@ -73,6 +73,20 @@ const ALL_GENERATORS = {
   generateViewConfigJs: generateViewConfigJs.generate,
 };
 
+type FilesOutput = Map<string, string>;
+
+type GenerateFunction = (
+  libraryName: string,
+  schema: SchemaType,
+  packageName?: string,
+  assumeNonnull: boolean,
+  headerPrefix?: string,
+) => FilesOutput;
+
+type LibraryGeneratorsFunctions = $ReadOnly<{
+  [string]: Array<GenerateFunction>,
+}>;
+
 type LibraryOptions = $ReadOnly<{
   libraryName: string,
   schema: SchemaType,
@@ -113,7 +127,7 @@ type SchemasConfig = $ReadOnly<{
   test?: boolean,
 }>;
 
-const LIBRARY_GENERATORS = {
+const LIBRARY_GENERATORS: LibraryGeneratorsFunctions = {
   descriptors: [
     generateComponentDescriptorCpp.generate,
     generateComponentDescriptorH.generate,