Reorganize the folder structure

[HoangNguyen689]

The current structure could be easier to understand.

• The src folder is unnecessary. The inside folders/files can be directly in the root folder.
• The backend and frontend are ambiguous. Maybe they're unnecessary, too.
• code_syntaxes → examples
• Put lexer, parser, interpreter, ... inside internals folder. ref
• Consider creating .artifacts to store the executable app.