Resolves#15358
With this change, `brew shellenv` will accept a shell name parameter to
override its default output (based on `/bin/ps`) for advanced use cases
such as with dotfile templating and caching with
[twpayne/chezmoi](https://github.com/twpayne/chezmoi/discussions/2971).
When running under `fish`, this is the output:
```console
$ brew shellenv pwsh
[System.Environment]::SetEnvironmentVariable('HOMEBREW_PREFIX','/opt/homebrew',[System.EnvironmentVariableTarget]::Process)
[System.Environment]::SetEnvironmentVariable('HOMEBREW_CELLAR','/opt/homebrew/Cellar',[System.EnvironmentVariableTarget]::Process)
[System.Environment]::SetEnvironmentVariable('HOMEBREW_REPOSITORY','/opt/homebrew',[System.EnvironmentVariableTarget]::Process)
[System.Environment]::SetEnvironmentVariable('PATH',$('/opt/homebrew/bin:/opt/homebrew/sbin:'+$ENV:PATH),[System.EnvironmentVariableTarget]::Process)
[System.Environment]::SetEnvironmentVariable('MANPATH',$('/opt/homebrew/share/man'+$(if(${ENV:MANPATH}){':'+${ENV:MANPATH}})+':'),[System.EnvironmentVariableTarget]::Process)
[System.Environment]::SetEnvironmentVariable('INFOPATH',$('/opt/homebrew/share/info'+$(if(${ENV:INFOPATH}){':'+${ENV:INFOPATH}})),[System.EnvironmentVariableTarget]::Process)
$ brew shellenv
set -gx HOMEBREW_PREFIX "/opt/homebrew";
set -gx HOMEBREW_CELLAR "/opt/homebrew/Cellar";
set -gx HOMEBREW_REPOSITORY "/opt/homebrew";
set -q PATH; or set PATH ''; set -gx PATH "/opt/homebrew/bin" "/opt/homebrew/sbin" $PATH;
set -q MANPATH; or set MANPATH ''; set -gx MANPATH "/opt/homebrew/share/man" $MANPATH;
set -q INFOPATH; or set INFOPATH ''; set -gx INFOPATH "/opt/homebrew/share/info" $INFOPATH;
```
The specific case presented in the mentioned discussion could be
mitigated by an additional level of indirection (`{{ output "fish" "-c"
"'brew shellenv'" | trim }}`), that requires that the running system
have the target shells installed, when they are not strictly necessary
with `brew shellenv`.
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