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/FileAPI/Blob-empty-constructor.txt

@@ -1 +1 @@
-PASS
+PASS

Tests/LibWeb/Text/expected/FileAPI/Blob-intialized-with-strings-read-from-arrayBuffer.txt

@@ -1 +1 @@
-PASS
+PASS

Tests/LibWeb/Text/expected/FileAPI/Blob-intialized-with-strings-read-from-text.txt

@@ -1 +1 @@
-PASS
+PASS