Handle no-init let-declared dependency identifier

Closes steadicat#4

Author: duncanbeevers

__tests__/require-usememo.ts

@@ -142,5 +142,13 @@ ruleTester.run("useMemo", rule, {
       options: [{ strict: true }],
       errors: [{ messageId: "unknown-usememo-props" }],
     },
+    {
+      code: `const Component = () => {
+        let myObject;
+        myObject = {};
+        return <Child prop={myObject} />;
+      }`,
+      errors: [{ messageId: "usememo-const" }],
+    },
   ],
 });

common.ts

@@ -53,6 +53,10 @@ function getIdentifierMemoStatus(
   if (node.type !== "VariableDeclarator") return MemoStatus.Memoized;
   if (node.parent.kind === "let") {
     context.report({ node, messageId: "usememo-const" });
+    if (!node.init) {
+      // Rely on usememo-const reported error to fail this identifier
+      return MemoStatus.Memoized;
+    }
   }
   return getExpressionMemoStatus(context, node.init);
 }