Add %template/%endtemplate pre-processor directives

[aspizu]

Adds %template and %endtemplate pre-processor directives that allow defining multi-line macro bodies as a named template, stored in simple_defines for later substitution.

Changes

• src/lexer/token.rs: Added Token::Template and Token::EndTemplate variants with Display implementations
• src/pre_processor.rs: Added parse_template() method and updated token cleanup filter

Re-implementation of feature/template-directive based from latest main.

[faretek1]

Isnt it dangerous to use u8 here because of unicode?