Fix incorrect tokenization of SCSS class extensions

[croccifixio]

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions

Description of the Change

This PR prevents extended SCSS class names with 2 or more hyphens after the name break from being tokenized as variables. Consider the following code snippet:

.block {
   &__element {}
   &--modifier {}
   &.another-block {}
   &-wrapper {}
}

__element, .another-block and -wrapper are all tokenized as punctuation.definition.entity.css.

--modifier is incorrectly tokenized as variable.scss.

Alternate Designs

None.

Benefits

Proper token colorization.

Possible Drawbacks

None that I can foresee.

Applicable Issues

#241