Fix plugin-update --all when there are no plugins

Issue: when asdf is first installed, there are no plugins, and running `asdf plugin-update --all` crashes because the plugin directory doesn't exist; similarly, the command crashes if the plugins directory exists yet doesn't contain any subdirectories.

Solution: This patch changes the directory loop from using a wildcard `plugins/*` to a more robust approach: first verify the plugins directory exists, then use the`find` command to list subdirectories and correctly handle the corner case of no subdirectories.

Future: Consider printing an error message and/or advice message. Consider automatically creating the plugins directory as needed. Consider prompting the user to install some popular plugins.

Fixes #768

Fixes #690

fix: typo causing plugin-test to erroneously fail

fix shims folder path

make sure the shims folder is appended to path only once

Fix #756

fix: revert to cmds to precompute brew prefix on addition to shell config

Install one tool specified in .tool-versions

`asdf install <name>` installs that single tool at the version specified
in `.tool-versions`.  If there is no `.tool-versions` or if that tool is
not in it, an error message is shown and `asdf` exits `1`.

Implements #759

Docs: Clarify shim creation for writing plugins