Code clean-up: renaming, adding comments (where required), used some C++20 features.

Add .vscode to gitignore.

Simplify ServerApp CMakeLists.

Comment server main function.

Add test to "CTest" list.

Clean up MessageBuilder.

Use std::format function (C++20 feature) in UsersDatabase.

Update googletest library.

Update to C++20 standard.

Renaming in UserDatabase with new convention.

Rename DB to Database.

Renaming source directory to src. Add CMakeLists.txt to ext and src directory.

Move to Qt6.

Fix after pull.

Refactored server and client code into its own files.