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/setter/associativity.wren

@@ -13,6 +13,6 @@ var c = Foo.new("c")
 
 // Assignment is right-associative.
 a.bar = b.bar = c.bar = "d"
-IO.print(a.toString) // expect: d
-IO.print(b.toString) // expect: d
-IO.print(c.toString) // expect: d
+System.print(a.toString) // expect: d
+System.print(b.toString) // expect: d
+System.print(c.toString) // expect: d

test/language/setter/instance.wren

@@ -2,7 +2,7 @@ class Foo {
   construct new() {}
 
   bar=(value) {
-    IO.print(value)
+    System.print(value)
   }
 }
 

test/language/setter/result.wren

@@ -4,4 +4,4 @@ class Foo {
 }
 
 var foo = Foo.new()
-IO.print(foo.bar = "value") // expect: result
+System.print(foo.bar = "value") // expect: result

test/language/setter/same_name_as_method.wren

@@ -1,7 +1,7 @@
 class Foo {
   construct new() {}
-  bar=(value) { IO.print("set") }
-  bar { IO.print("get") }
+  bar=(value) { System.print("set") }
+  bar { System.print("get") }
 }
 
 var foo = Foo.new()

test/language/setter/static.wren

@@ -1,6 +1,6 @@
 class Foo {
   static bar=(value) {
-    IO.print(value)
+    System.print(value)
   }
 }