completions(bash, fish): tweak formatting

[ee7]

Currently, the completions aren't auto-generated from a Nim source of
truth. Make it easier to scan the fish completions by making them more
similar to the output of configlet --help:

• Group __fish_use_subcommand completions.
• Order subcommands and options alphabetically.
• Align -s, -l, and -d.
• Always put -xa on its own line.

Also, format the bash completions with shfmt:

shfmt --indent 2 --case-indent --space-redirects --keep-padding

---

This PR is a bit bikeshed-y, but I claim that the new configlet.fish is more readable than the old configlet.fish. So I think this is marginally worthwhile, at least until we close #638.

The extra whitespace doesn't add binary bloat because we compress the completion scripts at compile time and uncompress them at runtime (see #652).

The configlet help message:

$ configlet --help
[...]
Commands:
  completion  Output a completion script for a given shell
  fmt         Format the exercise '.meta/config.json' files
  generate    Generate Concept Exercise 'introduction.md' files from 'introduction.md.tpl' files
  info        Print some information about the track
  lint        Check the track configuration for correctness
  sync        Check or update Practice Exercise docs, metadata, and tests from 'problem-specifications'.
              Check or populate missing 'files' values for Concept/Practice Exercises from the track 'config.json'.
  uuid        Output new (version 4) UUIDs, suitable for the value of a 'uuid' key

Options for completion:
  -s, --shell <shell>          Choose the shell type (required)
                               Allowed values: b[ash], f[ish], z[sh]

Options for fmt:
  -e, --exercise <slug>        Only operate on this exercise
  -u, --update                 Prompt to write formatted files
  -y, --yes                    Auto-confirm the prompt from --update

Options for info:
  -o, --offline                Do not update the cached 'problem-specifications' data

Options for sync:
  -e, --exercise <slug>        Only operate on this exercise
  -o, --offline                Do not update the cached 'problem-specifications' data
  -u, --update                 Prompt to update the unsynced track data
  -y, --yes                    Auto-confirm prompts from --update for updating docs, filepaths, and metadata
      --docs                   Sync Practice Exercise '.docs/introduction.md' and '.docs/instructions.md' files
      --filepaths              Populate empty 'files' values in Concept/Practice exercise '.meta/config.json' files
      --metadata               Sync Practice Exercise '.meta/config.json' metadata values
      --tests [mode]           Sync Practice Exercise '.meta/tests.toml' files.
                               The mode value specifies how missing tests are handled when using --update.
                               Allowed values: c[hoose], i[nclude], e[xclude] (default: choose)

Options for uuid:
  -n, --num <int>              Number of UUIDs to output

Global options:
  -h, --help                   Show this help message and exit
      --version                Show this tool's version information and exit
  -t, --track-dir <dir>        Specify a track directory to use instead of the current directory
  -v, --verbosity <verbosity>  The verbosity of output.
                               Allowed values: q[uiet], n[ormal], d[etailed] (default: normal)