fix(bash): improve integration with bash-preexec

[davidkna]

Description

1. For bash-preexec detection, a check for the official bash_preexec_imported variable in addition to the private __bp_imported for prior versions (see Library Authors documentation can still lead to issues rcaloras/bash-preexec#146 and Doesn't work with bash in login mode #5686) I also removed the check for the explicit value defined since that matches the official sample check more strongly.
2. Always use BP_PIPESTATUS if its length is greater than 0, because the value of PIPESTATUS could be unpredictable (feat(bash): Support right prompt and transience #4902 (comment)).

Motivation and Context

Closes #

Screenshots (if appropriate):

How Has This Been Tested?

• I have tested using MacOS
• I have tested using Linux
• I have tested using Windows

Checklist:

• I have updated the documentation accordingly.
• I have updated the tests accordingly.

[davidkna]

Thanks for the review @andytom!