feat: default to universal inferred context selector

[idoros]

This PR changes the inferred selector context under the new experimentalSelectorInference flag.

The default inferred selector context is used in 2 cases

• the starting point for any selector
• reset after a combinator (with experimentalSelectorInference)

Before this change the inferred selector used was the stylesheet root, and this PR changes it to be a universal selector (behind the flag), which means that custom-pseudo-element/class cannot be floating:

.root {
    -st-states: state;
}
.part {}

/* all of these cases will now report unknown pseudo diagnostics and will not transform */
:state {}
::part {}
.x :state {}
.x ::part {}