[🔥AUDIT🔥] [rg.audit.2] Add flowgen type defs (#618)

🖍 _This is an audit!_ 🖍

## Summary:
Added flowgen.d.ts to fix typecheck.

Issue: FEI-4960

## Test plan:
`yarn typecheck`

Author: somewhatabstract

Auditors: #typescript, #frontend-infra

Required Reviewers:

Approved By:

Checks: ⌛ Lint, typecheck, and coverage check (ubuntu-latest, 16.x), ✅ Prime node_modules cache for primary configuration (ubuntu-latest, 16.x), ✅ gerald, ⏭  dependabot, ⌛ Analyze (javascript)

Pull Request URL: #618

.changeset/breezy-baboons-play.md

@@ -0,0 +1,2 @@
+---
+---

build-settings/flowgen.d.ts

@@ -0,0 +1,39 @@
+// Adapted from https://github.com/joarwilk/flowgen/blob/master/index.js.flow
+
+declare module "flowgen" {
+    export type Options = {
+        jsdoc?: boolean;
+        interfaceRecords?: boolean;
+        moduleExports?: boolean;
+        quiet?: boolean;
+        inexact?: boolean;
+    };
+
+    export type Compiler = {
+        compileTest(path: string, target: string): void;
+        compileDefinitionString(
+            string: string,
+            options?: Options,
+            mapSourceCode?: (
+                source: string | void,
+                fileName: string,
+            ) => string | void,
+        ): string;
+        compileDefinitionFile(
+            path: string,
+            options?: Options,
+            mapSourceCode?: (
+                source: string | void,
+                fileName: string,
+            ) => string | void,
+        ): string;
+
+        // Low-level exports
+        reset(options?: Options): void;
+        setChecker(checker: any /* ts.TypeChecker */): void;
+        compile(sourceFile: any /* ts.SourceFile */): string;
+    };
+
+    export function beautify(str: string): string;
+    export const compiler: Compiler;
+}