Simplify bash PROMPT_COMMAND handling

[Daniel Imms (Tyriar)]

This change attempts to evaluate PROMPT_COMMAND in the same way that
bash would, adding support or many more cases and removing the check
that disables unsupported PROMPT_COMMANDS.

Fixes #155221
Fixes #146197

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Tested true/false exit codes with:

• Simple PS1
• PROMPT_COMMAND echo
• PROMPT_COMMAND function
• PROMPT_COMMAND ;
• PROMPT COMMAND &&
• Starship
• oh-my-posh
• git-bash-prompt
• preexec integration

It's possible PROMPT_COMMAND of the form ("first" "second") aren't working correctly, but I didn't totally understand how it's meant to be handled as no shell integration only seems to evaluate "first" in that case which we do.

FYI Megan Rogge (@meganrogge)

[Megan Rogge (meganrogge)]

Yep I was aware that other terminals with shell integration also don't support that kind of valid syntax. This looks much simpler, so probably not worth the added complexity to support it