Log Message:
shells/yash: update to 2.57

# Noteworthy changes

* Added support for the "$POST_PROMPT_COMMAND" variable, whose value
 is executed after reading a command line in the interactive shell.

* If the shell exits because of a shell error during the EXIT trap,
  the shell now returns the exit status of the error rather than that
  of the last command before the EXIT trap.

* [line-editing] Fixed the spurious error message printed when
  completing after git config alias. with the nounset shell option
  enabled.

* [line-editing] Completion no longer inserts a redundant backslash
  to escape a character included in the completed word
  when the cursor follows another backslash.
  Updated the sample initialization script (yashrc):

* Added setup for VS Code shell integration.