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/language/return/after_else.wren

@@ -1,3 +1,3 @@
-IO.print(Fn.new {
+System.print(Fn.new {
   if (false) "no" else return "ok"
 }.call()) // expect: ok

test/language/return/after_if.wren

@@ -1,3 +1,3 @@
-IO.print(Fn.new {
+System.print(Fn.new {
   if (true) return "ok"
 }.call()) // expect: ok

test/language/return/after_while.wren

@@ -1,3 +1,3 @@
-IO.print(Fn.new {
+System.print(Fn.new {
   while (true) return "ok"
 }.call()) // expect: ok

test/language/return/in_function.wren

@@ -1,6 +1,6 @@
 var f = Fn.new {
   return "ok"
-  IO.print("bad")
+  System.print("bad")
 }
 
-IO.print(f.call()) // expect: ok
+System.print(f.call()) // expect: ok

test/language/return/in_method.wren

@@ -3,8 +3,8 @@ class Foo {
 
   method {
     return "ok"
-    IO.print("bad")
+    System.print("bad")
   }
 }
 
-IO.print(Foo.new().method) // expect: ok
+System.print(Foo.new().method) // expect: ok

test/language/return/return_null_if_newline.wren

@@ -1,6 +1,6 @@
 var f = Fn.new {
   return
-  IO.print("bad")
+  System.print("bad")
 }
 
-IO.print(f.call()) // expect: null
+System.print(f.call()) // expect: null