style: Hide LocalMatchingContext.

This type is a lot of complexity related to a very specific thing such as the
hover and active quirk.

Instead of that, move `nesting_level` to `MatchingContext`, and simplify all
this computing whether the quirk applies upfront, for each complex selector we
test.

This is less error-prone, and also allows simplifying more stuff in a bit.

components/selectors/context.rs

@@ -102,6 +102,9 @@ pub struct MatchingContext<'a> {
     /// See https://drafts.csswg.org/selectors-4/#scope-pseudo
     pub scope_element: Option<OpaqueElement>,
 
+    /// The current nesting level of selectors that we're matching.
+    pub nesting_level: usize,
+
     quirks_mode: QuirksMode,
     classes_and_ids_case_sensitivity: CaseSensitivity,
 }
@@ -140,6 +143,7 @@ impl<'a> MatchingContext<'a> {
             relevant_link_found: false,
             classes_and_ids_case_sensitivity: quirks_mode.classes_and_ids_case_sensitivity(),
             scope_element: None,
+            nesting_level: 0,
         }
     }