gz.in: sort list of commands alphabetically

[scpeters]

🦟 Bug fix

Improves usability of gz tool

Summary

Currently the order of commands listed when running gz or gz help is a bit random, for example:

List of available commands:

  help:          Print this help text.
  plugin:        Print information about plugins.
  sdf:           Utilities for SDF files.
  gui:           Launch graphical interfaces.
  fuel:          Manage simulation resources.
  launch:        Run and manage executables and plugins.
  model:         Print information about models.
  sim:           Run and manage the Gazebo Simulator.
  param:         List, get or set parameters.
  gazebo:        Deprecated. Alias for sim.
  topic:         Print information about topics.
  service:       Print information about services.
  log:           Record or playback topics.
  msg:           Print information about messages.

Note that the help command is always listed first, but I've added some alphabetization of the non-help commands in this pull request:

List of available commands:

  help:          Print this help text.
  fuel:          Manage simulation resources.
  gazebo:        Deprecated. Alias for sim.
  gui:           Launch graphical interfaces.
  launch:        Run and manage executables and plugins.
  log:           Record or playback topics.
  model:         Print information about models.
  msg:           Print information about messages.
  param:         List, get or set parameters.
  plugin:        Print information about plugins.
  sdf:           Utilities for SDF files.
  service:       Print information about services.
  sim:           Run and manage the Gazebo Simulator.
  topic:         Print information about topics.

Checklist

• [X] Signed all commits for DCO
• [ ] Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [X] codecheck passed (See contributing)
• [X] All tests passed (See test coverage)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.