Add src-rust-lib and clean src-tauri

[maidh91]

Context

• This is one step in the plan to support this app running on web browser, and mobile
• Goal
  • in Desktop mode, src/commands will calls to @tauri-apps
  • in Web browser mode, src/commands will calls to some backend
    • eventually, the dir structure can be like this

wealthfolio/
├── src/
├── src-tauri/
├── src-rust-api/    # some REST api backend
├── src-rust-lib/    # shared rust code using `src-tauri` and `src-rust-api` 

Summary

• This src-rust-lib contains shared code using in different project like src-tauri (desktop) or src-rust-api (REST api)