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.

Tests/LibWeb/Text/expected/get-bounding-client-rect.txt

@@ -1,2 +1,2 @@
  inline   {"x":8,"y":500,"width":784,"height":150,"top":500,"right":792,"bottom":650,"left":8}
-{"x":8,"y":650,"width":41.296875,"height":17.46875,"top":650,"right":49.296875,"bottom":667.46875,"left":8}
+{"x":8,"y":650,"width":41.296875,"height":17,"top":650,"right":49.296875,"bottom":667,"left":8}