Introduce Mono Repo Structure

[devleejb]

What would you like to be added:
Currently, the CodePair project consists of a React frontend and a NestJS backend. I propose implementing a mono repo structure using tools like pnpm or Turborepo. This will enhance the project's organization and streamline dependency management across both the frontend and backend components.

I suggest referencing the implementation of pnpm in the yorkie-js-sdk project as a potential guideline: Yorkie JS SDK Issue #648.

Why is this needed:
Adopting a mono repo structure can simplify the development workflow, improve the consistency of dependency versions, and facilitate cross-team collaboration. By aligning the project under a single repository, teams can increase their efficiency and maintainability of the codebase.

[choidabom]

This issue is similar to #299. Can I try this?

[devleejb]

@choidabom Sure! I have assigned you!