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/string/count.wren

@@ -1,18 +1,18 @@
-IO.print("".count)   // expect: 0
-IO.print("a string".count) // expect: 8
+System.print("".count)   // expect: 0
+System.print("a string".count) // expect: 8
 
 // 8-bit clean.
-IO.print("\0".count)  // expect: 1
-IO.print("a\0b".count)  // expect: 3
-IO.print("\0c".count)  // expect: 2
-IO.print(("a\0b" + "\0c").count)  // expect: 5
+System.print("\0".count)  // expect: 1
+System.print("a\0b".count)  // expect: 3
+System.print("\0c".count)  // expect: 2
+System.print(("a\0b" + "\0c").count)  // expect: 5
 
 // Treats a UTF-8 sequence as a single item.
 //
 // Bytes:           11111
 //        012345678901234
 // Chars: sø mé ஃ  thî ng
-IO.print("søméஃthîng".count) // expect: 10
+System.print("søméஃthîng".count) // expect: 10
 
 // Counts invalid UTF-8 one byte at a time.
-IO.print("\xefok\xf7".count) // expect: 4
+System.print("\xefok\xf7".count) // expect: 4