Insert keyword and access modifier explicitly

[lizable]

Is your feature request related to a problem? Please describe. A clear and concise description of what the problem is. Ex. I'm always frustrated when [...] 기능 요청이 특정 문제에 연관된 것이라면 여기에 그 문제를 설명해주세요. For now, classes declared in most of the components are inherited from backend.ai-page component, which is primarily expanded from "LitElement" class. The problem is that there are not enough guidelines or any differences between "overridden" functions and "class-specific defined" functions; bringing confusing situations or such as using "overridden" functions without any cautions. Also, there's no "explicit" definition for a function used only inside the class, which makes contributors easily abuse those functions outside of class.

Describe the solution you'd like A clear and concise description of what you want to happen. 어떤 기능이 있으면 좋겠는지 자세히 설명해주세요. There are two things to resolve in this issue:

⚠️  NOTE: Some of the features are not completely applied to browsers. (e.g. override keyword is in Stage 3.) Therefore you may need to check whether it works properly on former versions of browsers.

• [ ] Use override keyword prefix when it's overridden from superclass.
• [ ] Provide precise member visibility on both member variables and member functions explicitly
  • [ ] Use proper access modifiers (if needed)
  • [ ] Update tsconfig to raise an error when omitting _(underscore) starts in private function or private variable or vice-versa.

Describe alternatives you've considered A clear and concise description of any alternative solutions or features you've considered. 혹시 다른 대안들을 생각해본 적이 있다면 함께 적어주세요. If possible, we could add about this in our new code convention.

Additional context Add any other context or screenshots about the feature request here. 기능 요청에 대해 보다 잘 이해할 수 있는 다른 맥락을 기술해주세요.