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/modules.markdown

@@ -173,7 +173,7 @@ like:
     import "shared"
 
     // shared.wren
-    IO.print("Shared!")
+    System.print("Shared!")
 
 Here, "a" and "b" both want to use "shared". If "shared" defines some top-level
 state, we only want a single copy of that in memory. To handle this, a module's
@@ -211,14 +211,14 @@ For example:
     import "a"
 
     // a.wren
-    IO.print("start a")
+    System.print("start a")
     import "b"
-    IO.print("end a")
+    System.print("end a")
 
     // b.wren
-    IO.print("start b")
+    System.print("start b")
     import "a"
-    IO.print("end b")
+    System.print("end b")
 
 This program runs successfully and prints: