Add a custom Pygments lexer plug-in for Wren syntax highlighting, update contributing docs with setup instructions for the lexer, and switch every `:::dart` annotation across all doc/site markdown files to `:::wren` so code examples render with the correct language grammar.
- Delete wren_io.c, wren_io.h, and io.wren source files
- Remove multi-arity print overloads and IO.read/IO.time methods
- Add System class with print, printAll, write, writeAll methods to core.wren
- Update all documentation examples and README to use System.print instead of IO.print
Create new modules.markdown guide explaining Wren's module system, import syntax, and top-level scoping. Update error-handling.markdown to use "Top-level variable" instead of "Global variable" in error messages. Revise variables.markdown to clarify top-level variable visibility and link to the new modules page. Add "Modules" link to the site navigation template.
