[#1] fix: 'React' refers to a UMD global, but the current file is a m…

…odule error fixed by importing React & tsconfig.json.compilerOptions.jsx = react-jsx -> react

src/Root.tsx

@@ -1,8 +1,9 @@
-import { StrictMode } from 'react';
-import { Router } from 'react-router-dom';
-import { RecoilRoot } from 'recoil';
-import App from 'components/App';
-import { historySingleton } from 'singleton/history';
+import React from "react";
+import { StrictMode } from "react";
+import { Router } from "react-router-dom";
+import { RecoilRoot } from "recoil";
+import App from "components/App";
+import { historySingleton } from "singleton/history";
 
 const Root = () => {
   return (

src/components/App.tsx

@@ -1,3 +1,4 @@
+import React from "react";
 import GreetingPage from "pages/GreetingPage";
 import { Route, Switch } from "react-router-dom";
 import GlobalStyle from "styles/GlobalStyle";