[feat] user type 정의

i-prefix를 붙이는 것에 대하여 더 공부할 것..!
microsoft/TypeScript-Handbook#121
https://stackoverflow.com/questions/31876947/confused-about-the-interface-and-class-coding-guidelines-for-typescript/41967120#41967120

Author: lallaheeee

src/types/index.ts

@@ -0,0 +1,6 @@
+export interface IUser {
+    githubId: string;
+    email: string;
+    username: string;
+    avatar: string;
+}