feat: replace IO class with System class and remove separate IO module

- 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

test/core/range/reduce.wren

@@ -1,4 +1,4 @@
 var range = 1..10
 
-IO.print(range.reduce {|a, b| a + b }) // expect: 55
-IO.print(range.reduce(100) {|a, b| a < b ? a : b }) // expect: 1
+System.print(range.reduce {|a, b| a + b }) // expect: 55
+System.print(range.reduce(100) {|a, b| a < b ? a : b }) // expect: 1