Properly support multiline help string and fix broken fish completion from #3126

[sitiyou]

See #3502 for details.

This PR reverts the changes introduced in #3126 and properly handles multi-line help strings by quoting \n and replacing \t with a space.

According to the Fish documentation:

Command substitutions found in ARGUMENTS should return a newline-
separated list of arguments, and each argument may optionally have a tab
character followed by the argument description.

fix #3043 and #3502

[kdeldycke]

OK so #3126 was a PR introducing a regression that was merge too early and broke fish completion in 8.4.0. The fix was naive as newlines can't serve double duty as both item and field separator in fish, as @sitiyou just pointed out.

This PR apply the right sanitation proposed in #3126 of escaping newline characters \n.

The deeper issue is that nowhere in Click do we run fish code with fish. But that's another work entirely.

So this is good to me to be merged.

[kdeldycke]

Arf. I messed up and merged this issue to main instead of stable. Will fix it.

[kdeldycke]

A fix for the wrong merge target is being worked on at #3508