Infer short completion descriptions for commandline flags

[PJungkamp]

Motivation

Descriptions for commandline flags may not include newlines and should be rather short for display in a shell.
The current completion code asserts that a description does not contain newlines.
Current descriptions for e.g. -I trigger that assertion and break commandline flag completion.

Context

See #7792

Here I shorten the description by truncating the string on '\n' or '.' to and add an ellipsis ' [...]' if sensible.
I chose to split on \n as well as . to keep the descriptions a single sentence.
I also had to trim at least an empty line from the front of the string to make -I work.

This yields something like the following for fish:

$ nix shell -
-c                                     (Command and arguments to be executed, defaulting to `$SHELL`)
-f  (Interpret installables as attribute paths relative to the Nix expression stored in *file* [...])
-I                                                          (Add *path* to the Nix search path [...])
-i                              (Clear the entire environment (except those specified with `--keep`))
-j                                                            (The maximum number of parallel builds)
-k                                                             (Keep the environment variable *name*)
-L                                                          (Print full build logs on standard error)
-u                                                            (Unset the environment variable *name*)
-v                                                             (Increase the logging verbosity level)
--                                                                                                   

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or incompatible change: updated release notes