feat(cli): add shell completions - #14341

Adds a command to install shell completions for the detected or otherwise chosen shell, coder completion.

• Enum and enum array flags now display their type in help commands and auto-generated documentation as Val1|Val2|... and [Val1|Val2|...] respectively, instead of just string or string-array.
• To reuse a dependency introduced in serpent, the configssh file is replaced using WriteFile from natefinch/atomic.
• Table columns struct tags have been unified to always use spaces over underscores. The table formatter previously accepted both, but always displayed the headers using spaces. The change was made to support providing completions, and validating, the --column flag on commands with a table output.

Future

• Use a new coder/serpent release instead of a specific commit.

Closes #13502.