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/language/string/byte_escapes.wren

@@ -1,12 +1,12 @@
 var bytes = "\x00\x12\x34\x56\x78\xab\xCD\xfFf".bytes
 
-IO.print(bytes[0]) // expect: 0
-IO.print(bytes[1]) // expect: 18
-IO.print(bytes[2]) // expect: 52
-IO.print(bytes[3]) // expect: 86
-IO.print(bytes[4]) // expect: 120
-IO.print(bytes[5]) // expect: 171
-IO.print(bytes[6]) // expect: 205
-IO.print(bytes[7]) // expect: 255
+System.print(bytes[0]) // expect: 0
+System.print(bytes[1]) // expect: 18
+System.print(bytes[2]) // expect: 52
+System.print(bytes[3]) // expect: 86
+System.print(bytes[4]) // expect: 120
+System.print(bytes[5]) // expect: 171
+System.print(bytes[6]) // expect: 205
+System.print(bytes[7]) // expect: 255
 // "f".
-IO.print(bytes[8]) // expect: 102
+System.print(bytes[8]) // expect: 102