Emit function param and return type annotations

compiler/packages/babel-plugin-react-forget/src/HIR/BuildHIR.ts

@@ -207,6 +207,7 @@ export function lower(
   return Ok({
     id,
     params,
+    returnType: null, // TODO: extract the actual return type node if present
     body: builder.build(),
     context,
     generator: func.node.generator === true,

compiler/packages/babel-plugin-react-forget/src/HIR/HIR.ts

@@ -233,6 +233,7 @@ export type HIRFunction = {
   id: string | null;
   env: Environment;
   params: Array<Place | SpreadPattern>;
+  returnType: t.FlowType | t.TSType | null;
   context: Array<Place>;
   body: HIR;
   generator: boolean;