Fix patterns for variables like "$x1"

[xi]

Take for example the variable pattern: ^[_]?[a-z]+([a-z0-9-]+[a-z0-9]+)?$.

• If ([a-z0-9-]+[a-z0-9]+)? is not matched it allows an arbitrary number of lowercase alphabetic character with an optional underscore at the beginning.
• If ([a-z0-9-]+[a-z0-9]+)? is matched it does also allow numerics and dashes after that (but no dash at the end). But it will only match if there are at least two of these characters.

I assume this last bit is unintentional, as it prohibits variable names such as $x1 while allowing $x, $xs, and $x10. Apart from fixing that, I also removed some dispensable + as I think that makes the patterns easier to understand.

[bjankord]

Looks good to me, thanks for the PR @xi

[bjankord]

This has been released in https://github.com/bjankord/stylelint-config-sass-guidelines/releases/tag/v5.4.0