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/join.wren

@@ -1,10 +1,10 @@
 var str = "string"
 
-IO.print(str.join("") == str) // expect: true
+System.print(str.join("") == str) // expect: true
 
-IO.print(str.join(", ")) // expect: s, t, r, i, n, g
+System.print(str.join(", ")) // expect: s, t, r, i, n, g
 
 // 8-bit clean.
 var ing = "a\0b\0c"
-IO.print(ing.join("") == ing) // expect: true
-IO.print(ing.join(", ") == "a, \0, b, \0, c") // expect: true
+System.print(ing.join("") == ing) // expect: true
+System.print(ing.join(", ") == "a, \0, b, \0, c") // expect: true