Expand variables in the default syntax ${...:-$VAR}

[randoragon]

As the title says. Expanding variables in fallback values is standard POSIX behavior that's implemented in virtually all Linux shells (dash, bash, zsh, and many more I imagine). It is particularly useful for accessing XDG directories (my exact use case), e.g. "${XDG_DATA_HOME:-$HOME/.local/share}/". Would be great to have this.

Minimal reproducible example

fn main() {
    println!("{}", shellexpand::full("${NON_EXISTENT_VAR:-$HOME}/").unwrap());
}

expected output: /home/user/
actual output: $HOME/

Result with dash/bash/zsh

$ echo ${NON_EXISTENT_VAR:-$HOME}/
/home/user/

Technically this is recursive, i.e. echo ${ABC:-${DEF:-ghi}} in dash yields ghi if both $ABC and $DEF are undefined. But I understand that perfect POSIX-compliance might be too great an endeavor. IMO just a single-level expansion would be fine. Also an appropriate comment in the docs would be welcome.