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/core/num.markdown

@@ -132,9 +132,9 @@ from the beginning number to the ending number.
 
     :::dart
     var range = 1.2..3.4
-    IO.print(range.min)         // 1.2
-    IO.print(range.max)         // 3.4
-    IO.print(range.isInclusive) // true
+    System.print(range.min)         // 1.2
+    System.print(range.max)         // 3.4
+    System.print(range.isInclusive) // true
 
 ### **...**(other) operator
 
@@ -143,6 +143,6 @@ from the beginning number to the ending number not including the ending number.
 
     :::dart
     var range = 1.2...3.4
-    IO.print(range.min)         // 1.2
-    IO.print(range.max)         // 3.4
-    IO.print(range.isInclusive) // false
+    System.print(range.min)         // 1.2
+    System.print(range.max)         // 3.4
+    System.print(range.isInclusive) // false