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/Layout/expected/table/table-align-center-with-margin.txt

@@ -5,9 +5,9 @@ Viewport <#document> at (0,0) content-size 800x600 children: not-inline
         Box <table> at (109,101) content-size 87.34375x202 table-box [TFC] children: not-inline
           Box <tbody> at (109,101) content-size 83.34375x198 table-row-group children: not-inline
             Box <tr> at (111,103) content-size 83.34375x198 table-row children: not-inline
-              BlockContainer <td> at (112,193.265625) content-size 81.34375x17.46875 table-cell [BFC] children: inline
-                line 0 width: 81.34375, height: 17.46875, bottom: 17.46875, baseline: 13.53125
-                  frag 0 from TextNode start: 0, length: 10, rect: [112,193.265625 81.34375x17.46875]
+              BlockContainer <td> at (112,193.5) content-size 81.34375x17 table-cell [BFC] children: inline
+                line 0 width: 81.34375, height: 17, bottom: 17, baseline: 13.296875
+                  frag 0 from TextNode start: 0, length: 10, rect: [112,193.5 81.34375x17]
                     "off-center"
                 TextNode <#text>
               BlockContainer <(anonymous)> (not painted) children: inline