OSX `sed` is not GNU `sed`, and doesn't support the `\s` whitespace character sequence. This results in autocompletion suggestions including leading spaces, e.g.,
```
$ asdf local python \ \ 3.8.0
```
Using the POSIX compliant `[[:space:]]` sequence is more compatible, and removes the unnecessary spaces on OSX.
Add a "shell" command similar to the existing "global" and "local"
commands, which sets the version in an environment variable instead of
writing it to a file. This was inspired by the similar functionality in
rbenv.
It works by adding a wrapper function for the asdf command. It forwards
to a "sh-shell" command that returns the exports as shell code which is
then evaled by the wrapper. This is a little gross, but we need to run
the code in the shell context in order to set variables.
Resolves#378
Attempting to tab-complete an asdf command would result in a deprecation warning being printed.
The --authoritative/-A and --unauthoritative/-u flags have been removed from the `complete` command.
Context: https://github.com/fish-shell/fish-shell/pull/3660
* Rename `asdf which` -> `asdf current`
* Output `set by $path` with current command
* Use dummy plugin in current_command test
* Hide "set by" message if derived from legacy file
