LibWeb: Bring CSS line-height closer to other engines

This patch makes a few changes to the way we calculate line-height:

- `line-height: normal` is now resolved using metrics from the used
  font (specifically, round(A + D + lineGap)).

- `line-height: calc(...)` is now resolved at style compute time.

- `line-height` values are now absolutized at style compute time.

As a consequence of the above, we no longer need to walk the DOM
ancestor chain looking for line-heights during style computation.
Instead, values are inherited, resolved and absolutized locally.

This is not only much faster, but also makes our line-height metrics
match those of other engines like Gecko and Blink.

Userland/Libraries/LibWeb/CSS/MediaQuery.cpp

@@ -169,7 +169,7 @@ bool MediaFeature::compare(HTML::Window const& window, MediaFeatureValue left, C
 
             auto const& initial_font = window.associated_document().style_computer().initial_font();
             Gfx::FontPixelMetrics const& initial_font_metrics = initial_font.pixel_metrics();
-            Length::FontMetrics font_metrics { initial_font.presentation_size(), initial_font_metrics, CSSPixels::nearest_value_for(initial_font_metrics.line_spacing()) };
+            Length::FontMetrics font_metrics { initial_font.presentation_size(), initial_font_metrics };
 
             left_px = left.length().to_px(viewport_rect, font_metrics, font_metrics);
             right_px = right.length().to_px(viewport_rect, font_metrics, font_metrics);