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/this/closure.wren

@@ -5,4 +5,4 @@ class Foo {
 }
 
 var closure = Foo.new().getClosure
-IO.print(closure.call()) // expect: Foo
+System.print(closure.call()) // expect: Foo

test/language/this/nested_class.wren

@@ -2,16 +2,16 @@ class Outer {
   construct new() {}
 
   method {
-    IO.print(this) // expect: Outer
+    System.print(this) // expect: Outer
 
     Fn.new {
-      IO.print(this) // expect: Outer
+      System.print(this) // expect: Outer
 
       class Inner {
         construct new() {}
 
         method {
-          IO.print(this) // expect: Inner
+          System.print(this) // expect: Inner
         }
         toString { "Inner" }
       }

test/language/this/nested_closure.wren

@@ -5,4 +5,4 @@ class Foo {
 }
 
 var closure = Foo.new().getClosure
-IO.print(closure.call().call().call()) // expect: Foo
+System.print(closure.call().call().call()) // expect: Foo

test/language/this/this_in_method.wren

@@ -4,4 +4,4 @@ class Foo {
   baz { "baz" }
 }
 
-IO.print(Foo.new().bar.baz) // expect: baz
+System.print(Foo.new().bar.baz) // expect: baz

test/language/this/this_in_static_method.wren

@@ -1,7 +1,7 @@
 class Foo {
   static test {
-    IO.print(this == Foo) // expect: true
-    IO.print(this.bar) // expect: bar
+    System.print(this == Foo) // expect: true
+    System.print(this.bar) // expect: bar
   }
 
   static bar { "bar" }