LibWeb: Resolve CSS font property from value list

The font property now resolves into its various parts:

- font-family
- font-weight
- font-size
- font-style
- line-height

The font-variant and font-stretch parts are left unparsed since LibWeb
doesn't know how to render those.

Added `fonts.html` as a test for various forms of `font` declarations,
based on the examples in the spec.

Userland/Libraries/LibWeb/CSS/Identifiers.json

@@ -76,6 +76,7 @@
   "context-menu",
   "copy",
   "crosshair",
+  "cursive",
   "dashed",
   "decimal",
   "decimal-leading-zero",
@@ -85,6 +86,7 @@
   "double",
   "e-resize",
   "ew-resize",
+  "fantasy",
   "fixed",
   "flex",
   "flex-start",
@@ -100,6 +102,7 @@
   "inline-block",
   "inset",
   "inside",
+  "italic",
   "justify",
   "large",
   "larger",
@@ -112,6 +115,7 @@
   "lower-latin",
   "lower-roman",
   "medium",
+  "monospace",
   "move",
   "ne-resize",
   "nesw-resize",
@@ -125,6 +129,7 @@
   "ns-resize",
   "nw-resize",
   "nwse-resize",
+  "oblique",
   "outset",
   "outside",
   "overline",
@@ -143,7 +148,9 @@
   "row",
   "row-resize",
   "row-reverse",
+  "sans-serif",
   "scroll",
+  "serif",
   "se-resize",
   "small",
   "smaller",
@@ -166,6 +173,10 @@
   "table-row",
   "table-row-group",
   "text",
+  "ui-monospace",
+  "ui-rounded",
+  "ui-sans-serif",
+  "ui-serif",
   "underline",
   "uppercase",
   "upper-alpha",