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/core/number/bitwise_lsh.wren

@@ -1,15 +1,15 @@
-IO.print(0 << 0) // expect: 0
-IO.print(1 << 0) // expect: 1
-IO.print(0 << 1) // expect: 0
-IO.print(1 << 1) // expect: 2
-IO.print(2863311530 << 1) // expect: 1431655764
-IO.print(4042322160 << 1) // expect: 3789677024
+System.print(0 << 0) // expect: 0
+System.print(1 << 0) // expect: 1
+System.print(0 << 1) // expect: 0
+System.print(1 << 1) // expect: 2
+System.print(2863311530 << 1) // expect: 1431655764
+System.print(4042322160 << 1) // expect: 3789677024
 
 // Max u32 value.
-IO.print(4294967295 << 1) // expect: 4294967294
+System.print(4294967295 << 1) // expect: 4294967294
 
 // Past max u32 value.
-IO.print(4294967296 << 1) // expect: 0
+System.print(4294967296 << 1) // expect: 0
 
 // TODO: Negative numbers.
 // TODO: Floating-point numbers.