- added `prelude`
- moved `ast` to `parser`
- moved `Lexer` from `crate::lexer::lexer::Lexer` to `crate::lexer::Lexer`
- renamed `LexerError` to `SyntaxError`
- moved `SyntaxError` to `token::error`
- implemented `Display` for `SyntaxError`
- add `tokens.txt` to integration tests
- removed all unit tests for `Lexer` ( everything is tested in integration tests )
- improved syntax error assertations
