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/number/minus.wren

@@ -1,8 +1,8 @@
 // Infix.
-IO.print(5 - 3)      // expect: 2
-IO.print(3.1 - 0.24) // expect: 2.86
-IO.print(3 - 2 - 1)  // expect: 0
+System.print(5 - 3)      // expect: 2
+System.print(3.1 - 0.24) // expect: 2.86
+System.print(3 - 2 - 1)  // expect: 0
 
 // Unary negation.
 var a = 3
-IO.print(-a) // expect: -3
+System.print(-a) // expect: -3