Add C function dup and dup2 posix to system/io (#15675)

* * Add handle to dup and dup2 posix as duplicate and duplicateTo in std/ioutils.
* Added small test & changelog entry

* Fixed import in tioutils
removed when isMainModule

* * Nest test inside block. Rename proc var -> let in captureStdout
* Renamed tmpfile to iotuils.txt
* Added block: # duplicate, duplicateTo
* Improved docstring
* Clean non-idiomatic code
* Added runnable examples

* rm 2 trailing space in expected output

* Made syntax prettier

* Runnable example: file in getTempDir()

* Tmp -> Temp

* Fixed runnableExamples on windows

changelog.md

@@ -6,6 +6,8 @@
 
 - `prelude` now works with the JavaScript target.
 
+- Added `ioutils` module containing `duplicate` and `duplicateTo` to duplicate `FileHandle` using C function `dup` and `dup2`.
+
 
 ## Language changes
 
@@ -18,4 +20,3 @@
 
 
 ## Tool changes
-