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

doc/site/values.markdown

@@ -57,11 +57,11 @@ A handful of escape characters are supported:
 A `\u` followed by four hex digits can be used to specify a Unicode code point:
 
     :::dart
-    IO.print("\u0041\u0b83\u00DE") // "AஃÞ"
+    System.print("\u0041\u0b83\u00DE") // "AஃÞ"
 
 A `\x` followed by two hex digits specifies a single unencoded byte:
 
-    IO.print("\x48\x69\x2e") // "Hi."
+    System.print("\x48\x69\x2e") // "Hi."
 
 Strings are instances of class [String](core/string.html).
 
@@ -90,7 +90,7 @@ example:
     :::dart
     var list = ["a", "b", "c", "d", "e"]
     var slice = list[1..3]
-    IO.print(slice) // ["b", "c", "d"]
+    System.print(slice) // ["b", "c", "d"]
 
 Their class is [Range](core/range.html).