fix: mixin of class from st-scope params

[idoros]

This PR fix a regression bug (worked in v4) that causes mixin that is defined as part of @st-scope params to be mixed incorrectly. The issue causes the original mixin class to be added as part of the selector instead of being replaced by the context selector.

For example:

@st-scope .mix {
    .x {}
}

.into {
    -st-mixin: mix;
}

/* should transforms into */
.into .x {}

/* but is incorrectly transformed into */
.into .mix .x {}