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/module/shared_import/a.wren

@@ -1,5 +1,5 @@
 // nontest
-IO.print("a")
+System.print("a")
 import "shared" for Shared
 var A = "a " + Shared
-IO.print("a done")
+System.print("a done")

test/language/module/shared_import/b.wren

@@ -1,5 +1,5 @@
 // nontest
-IO.print("b")
+System.print("b")
 import "shared" for Shared
 var B = "b " + Shared
-IO.print("b done")
+System.print("b done")

test/language/module/shared_import/shared.wren

@@ -1,3 +1,3 @@
 // nontest
-IO.print("shared")
+System.print("shared")
 var Shared = "shared"

test/language/module/shared_import/shared_import.wren

@@ -8,5 +8,5 @@ import "b" for B
 // expect: b
 // expect: b done
 
-IO.print(A) // expect: a shared
-IO.print(B) // expect: b shared
+System.print(A) // expect: a shared
+System.print(B) // expect: b shared