[ENHANCEMENT] Lexer improvements + bugfix

[Ranastra]

• fixed line and column counting to start from zero, '\n' now checked before whitespace
• simplified lexer errors
• return value of lex function changed to include the position of tokens to be used for error handling

[create-issue-branch[bot]]

Branch issue-35 created!

[ElBe-Plaq]

Looks pretty good. But please use one of the templates (https://github.com/I-Language-Development/I-language-rust/issues/new/choose) next time.

[ElBe-Plaq]

Reminds me, that I have to do the enhancement template.