style: Refactor and add infrastructure for font metrics in style.

This commit itself only moves things around and adds an extra parameter to the
`apply_declarations` function to eventually handle #14079 correctly.

Probably needs a more granular API to query fonts, á la nsFontMetrics, but
that's trivial to do once this is landed.

Then we should make the font provider mandatory, and implement the missing stylo
bits.

components/style/animation.rs

@@ -403,6 +403,7 @@ fn compute_style_for_animation_step(context: &SharedStyleContext,
                                                previous_style,
                                                /* cascade_info = */ None,
                                                context.error_reporter.clone(),
+                                               /* Metrics provider */ None,
                                                CascadeFlags::empty());
             computed
         }