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/shebang/shebang.wren

@@ -1,2 +1,2 @@
 #!/bin/wren
-IO.print("ok") // expect: ok
+System.print("ok") // expect: ok

test/language/shebang/shebang_at_other_line.wren

@@ -1,3 +1,3 @@
 // expect error line 3
-IO.print("nope")
+System.print("nope")
 #!/bin/wren

test/language/shebang/shebang_invalid.wren

@@ -1,3 +1,3 @@
 #/invalid/shebang
 // expect error line 1
-IO.print("nope")
+System.print("nope")