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/cos.wren

@@ -1,6 +1,6 @@
-IO.print(0.cos)             // expect: 1
-IO.print(Num.pi.cos)        // expect: -1
-IO.print((2 * Num.pi).cos)  // expect: 1
+System.print(0.cos)             // expect: 1
+System.print(Num.pi.cos)        // expect: -1
+System.print((2 * Num.pi).cos)  // expect: 1
 
 // this should of course be 0, but it's not that precise
-IO.print((Num.pi / 2).cos)  // expect: 6.1232339957368e-17
+System.print((Num.pi / 2).cos)  // expect: 6.1232339957368e-17