Feature Request: Minimalistic tab-completion with descriptions

[am11]

Recent improvements (see commit acae889 by @rmyorston]) added basic tab completion for builtins, functions, and aliases in ash.

It would be great to extend this with static registration of completions, allowing interactive CLI tools to provide context-aware completions similar to zsh and pwsh.

e.g. here is how the zsh completion for deno(1) looks like:

zsh / cat /opt/homebrew/share/zsh/site-functions/_deno 
#compdef deno

autoload -U is-at-least

_deno() {
    typeset -A opt_args
    typeset -a _arguments_options
    local ret=1

    if is-at-least 5.2; then
        _arguments_options=(-s -S -C)
    else
        _arguments_options=(-s -C)
    fi

    local context curcontext="$curcontext" state line
    _arguments "${_arguments_options[@]}" : \
'--no-check=[Skip type-checking. If the value of "remote" is supplied, diagnostic errors from remote modules will be ignored]' \
'--import-map=[Load import map file from local file or remote URL   Docs\: https\://docs.deno.com/runtime/manual/basics/import_maps]:FILE:_files' \
'--node-modules-dir=[Sets the node modules management mode for npm packages]' \
'--vendor=[Toggles local vendor folder usage for remote modules and a node_modules folder for npm packages]' \
'-c+[Configure different aspects of deno including TypeScript, linting, and code formatting   Typically the configuration file will be called \`deno.json\` or \`deno.jsonc\` and   automatically detected; in that case this flag is not necessary.>
'--config=[Configure different aspects of deno including TypeScript, linting, and code formatting   Typically the configuration file will be called \`deno.json\` or \`deno.jsonc\` and   automatically detected; in that case this flag is not nece>
'*-r+[Reload source code cache (recompile TypeScript)   no value                                                 Reload everything   jsr\:@std/http/file-server,jsr\:@std/assert/assert-equals  Reloads specific modules   npm\:                    >
'*--reload=[Reload source code cache (recompile TypeScript)   no value                                                 Reload everything   jsr\:@std/http/file-server,jsr\:@std/assert/assert-equals  Reloads specific modules   npm\:              >
'--lock=[Check the specified lock file. (If value is not provided, defaults to "./deno.lock")]' \
'--cert=[Load certificate authority from PEM encoded file]:FILE:_files' \
'--unsafely-ignore-certificate-errors=[DANGER\: Disables verification of TLS certificates]' \
'*-R+[Allow file system read access. Optionally specify allowed paths]' \
'*--allow-read=[Allow file system read access. Optionally specify allowed paths]' \
...

and tab completion on subcommand shows them with description:

Proposal for ash

• Introduce a directory (/etc/ash/completion.d/) where scripts can define completions.
• Format similar to zsh (option[Description]), parsed minimally.

I think static completion is more aligned with busybox's minimalistic approach than dynamic registration mechanism (which requires more utilities / phases). Static is basically: search all lines in completion file which begin with the subcommand and display them with description.

[am11]

cc @baronfel, this is related to dotnet/sdk#42416 for Alpine Linux' default shell ash.