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_not.wren

@@ -1,22 +1,22 @@
-IO.print(~0) // expect: 4294967295
-IO.print(~1) // expect: 4294967294
-IO.print(~23) // expect: 4294967272
+System.print(~0) // expect: 4294967295
+System.print(~1) // expect: 4294967294
+System.print(~23) // expect: 4294967272
 
 // Max u32 value.
-IO.print(~4294967295) // expect: 0
+System.print(~4294967295) // expect: 0
 
 // Past max u32 value.
-IO.print(~4294967296) // expect: 4294967295
+System.print(~4294967296) // expect: 4294967295
 
 // Negative numbers.
-IO.print(~-1) // expect: 0
-IO.print(~-123) // expect: 122
-IO.print(~-4294967294) // expect: 4294967293
-IO.print(~-4294967295) // expect: 4294967294
-IO.print(~-4294967296) // expect: 4294967295
+System.print(~-1) // expect: 0
+System.print(~-123) // expect: 122
+System.print(~-4294967294) // expect: 4294967293
+System.print(~-4294967295) // expect: 4294967294
+System.print(~-4294967296) // expect: 4294967295
 
 // Floating point values.
-IO.print(~1.23) // expect: 4294967294
-IO.print(~0.00123) // expect: 4294967295
-IO.print(~345.67) // expect: 4294966950
-IO.print(~-12.34) // expect: 11
+System.print(~1.23) // expect: 4294967294
+System.print(~0.00123) // expect: 4294967295
+System.print(~345.67) // expect: 4294966950
+System.print(~-12.34) // expect: 11