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/multiple_variables/module.wren

@@ -5,4 +5,4 @@ var Module3 = "from module three"
 var Module4 = "from module four"
 var Module5 = "from module five"
 
-IO.print("ran module")
+System.print("ran module")

test/language/module/multiple_variables/multiple_variables.wren

@@ -3,8 +3,8 @@ import "module" for Module1, Module2, Module3, Module4, Module5
 // Only execute module body once:
 // expect: ran module
 
-IO.print(Module1) // expect: from module one
-IO.print(Module2) // expect: from module two
-IO.print(Module3) // expect: from module three
-IO.print(Module4) // expect: from module four
-IO.print(Module5) // expect: from module five
+System.print(Module1) // expect: from module one
+System.print(Module2) // expect: from module two
+System.print(Module3) // expect: from module three
+System.print(Module4) // expect: from module four
+System.print(Module5) // expect: from module five