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/is/is.wren

@@ -1,33 +1,33 @@
-IO.print(Num is Class) // expect: true
-IO.print(true is Bool) // expect: true
-IO.print(Fn.new { 1 } is Fn) // expect: true
-IO.print(123 is Num) // expect: true
-IO.print(null is Null) // expect: true
-IO.print("s" is String) // expect: true
+System.print(Num is Class) // expect: true
+System.print(true is Bool) // expect: true
+System.print(Fn.new { 1 } is Fn) // expect: true
+System.print(123 is Num) // expect: true
+System.print(null is Null) // expect: true
+System.print("s" is String) // expect: true
 
-IO.print(Num is Bool) // expect: false
-IO.print(null is Class) // expect: false
-IO.print(true is Fn) // expect: false
-IO.print(Fn.new { 1 } is Num) // expect: false
-IO.print("s" is Null) // expect: false
-IO.print(123 is String) // expect: false
+System.print(Num is Bool) // expect: false
+System.print(null is Class) // expect: false
+System.print(true is Fn) // expect: false
+System.print(Fn.new { 1 } is Num) // expect: false
+System.print("s" is Null) // expect: false
+System.print(123 is String) // expect: false
 
 // Everything extends Object.
-IO.print(Num is Object) // expect: true
-IO.print(null is Object) // expect: true
-IO.print(true is Object) // expect: true
-IO.print(Fn.new { 1 } is Object) // expect: true
-IO.print("s" is Object) // expect: true
-IO.print(123 is Object) // expect: true
+System.print(Num is Object) // expect: true
+System.print(null is Object) // expect: true
+System.print(true is Object) // expect: true
+System.print(Fn.new { 1 } is Object) // expect: true
+System.print("s" is Object) // expect: true
+System.print(123 is Object) // expect: true
 
 // Classes extend Class.
-IO.print(Num is Class) // expect: true
-IO.print(null is Class) // expect: false
-IO.print(true is Class) // expect: false
-IO.print(Fn.new { 1 } is Class) // expect: false
-IO.print("s" is Class) // expect: false
-IO.print(123 is Class) // expect: false
+System.print(Num is Class) // expect: true
+System.print(null is Class) // expect: false
+System.print(true is Class) // expect: false
+System.print(Fn.new { 1 } is Class) // expect: false
+System.print("s" is Class) // expect: false
+System.print(123 is Class) // expect: false
 
 // Ignore newline after "is".
-IO.print(123 is
+System.print(123 is
   Num) // expect: true