Expansion of variables by completion

Related problem

https://github.com/nushell/nushell/issues/6696

Describe the solution you'd like

I find myself using environment variables with short names as shortcuts to various places in the file system. Using such variables doesn't work with completion right now, e.g. ls $DIR/* doesn't complete possible file names in $DIR.

It would be great if the use of such variables would either a) work with completion out of the box b) be auto-expanded on completion c) be expanded on demand (e.g. by using a configurable keypress).

Preference is a), b), c).

For a) and b), it would fit my personal use to explicitly have to declare such variables as "auto-expanded by completion".

Describe alternatives you've considered

Maybe this can be done by configuring a custom completer but that's not well documented at the moment.

Additional context and details

No response

Start by reading the related issue #6696 and investigating Nushell's completion behavior for commands such as ls $DIR/*. Review how custom completers are configured, since the issue notes that this is not well documented. Done means deciding and implementing a supported approach for completing file names through selected environment variables, with the preferred behavior working out of the box.