fix: cases with mixin moved compound selector

[idoros]

This PR fixes 2 issues:

1) ambiguity of a mixed-in CSS class and root class

The issue happens when a mixed class is compounded with .root:state (root class with custom-pseudo-class). The issue is that there is a bug in the process of mixin that loses the actual root symbol when a stylesheet class other then root is mixed:

mixin.st.css:

.root {
    -st-mixin: x;
}
.root:x.mix {
    color: green;
}

entry.st.css:

@st-import [mix] from "./mixin.st.css";

/* @rule[1] .entry__c.mixin__root.mixin--x */
.c {
    -st-mixin: mix;
}

valid selector: .entry__c.mixin__root.mixin--x
error selector: .entry__c.mixin__root:x // root symbol is "wrongly" replaced with mix class and :x custom-pseudo-state is unresolved

2) multiple compound selectors after mixin

When compound selectors are connected after mixin class, they need to move to the beginning of the selector with the mixin if it is moved:

mixin.st.css:

.root {
    -st-mixin: x;
}
.root.mix:mix-state {
    color: green;
}

entry.st.css:

@st-import [mix] from "./mixin.st.css";

/* @rule[1] .entry__c.mixin--mix-state.mixin__root */
.c {
    -st-mixin: mix;
}

valid selector: .entry__c.mixin--mix-state.mixin__root
error selector: .entry__c.mixin__root:mix-state // mix-state is not moved/transformed