chore: refactor project structure

[azjezz]

closes #143

• add 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

[ryangjchandler]

Tiny spelling mistake - otherwise perfect. Thanks for removing that lexer config crap, been meaning to get rid of it.

[azjezz]

i believe that ast should be moved within parser, it doesn't make sense to have ast in root, while token is in lexer.