Add file info to span during tokenizing and handle error properly

[cakevm]

This is a first step towards better error handling. The Lexer::new get additionally to the source the file to allow to add this info to the span. This can be refactored and simplified in a later step.

For invalid character now a proper error message is printed. Cases for imported files need to be fixed.

Before:

⠋ Compiling...thread 'main' panicked at crates/core/src/lib.rs:352:50:
called `Result::unwrap()` on an `Err` value: LexicalError { kind: InvalidCharacter('#'), span: Span { start: 2810, end: 2810, file: None } }
stack backtrace:...

After:

Error: Invalid Character: "#" 
-> src/Flux.huff:2810-2810
       |
  > 49 | #
       |