fix: regex validate plugin names on plugin add cmd

[jthegedus]

Summary

Apply regex validation to plugin names when asdf plugin add is used.

Regex is ^[a-zA-Z0-9_-]+$ and applied via grep --extended-regexp.

Fixes: #789

Other Information

^[a-zA-Z0-9-]$ was the original discussion. I have added _ as well as the regex will then cover all popular naming conventions (camelCase, snake_case, kebab-case, PascalCase and UPPER_SNAKE).

We may restrict this regex further in future because of platform independent handling of filesystem paths with varying cases as discussed in asdf-vm/asdf-plugins#28

[Stratus3D]

Is grep --extended-regexp supported on all platforms? Otherwise changes here look good to me.

[jthegedus]

I am not sure of the portability of grep --extended-regexp (grep -E, I prefer the clearer extended flag names), but we're already using it in a number of places, so this change doesn't increase the surface area of tools or flags used.

I think we just wait until someone flags grep -E as being an issue for their platform before attempting to remove the dependence.

[Stratus3D]

@jthegedus sounds good. I didn't realize core was already using grep -E.