LibWeb/CSS: Treat 'mask' as a longhand property

Before this change, an element masked with 'mask-image: url(...)' would
show the mask, but 'mask: url(...)' would not. On e.g. dialogic.nl it
would show white boxes instead of the actual images in the top
navigation bar. We still do not support many of the other mask
properties, but with this change at least the masks show up in both
cases.

Tests/LibWeb/Text/expected/css/CSSStyleDeclaration-all-supported-properties-and-default-values.txt

@@ -1,6 +1,6 @@
 All supported properties and their default values exposed from CSSStyleDeclaration from getComputedStyle:
 'cssText': ''
-'length': '220'
+'length': '219'
 'parentRule': 'null'
 'cssFloat': 'none'
 'WebkitAlignContent': 'normal'

Tests/LibWeb/Text/expected/css/CSSStyleDeclaration-has-indexed-property-getter.txt

@@ -156,70 +156,69 @@ All properties associated with getComputedStyle(document.body):
     "153": "margin-left",
     "154": "margin-right",
     "155": "margin-top",
-    "156": "mask",
-    "157": "mask-image",
-    "158": "mask-type",
-    "159": "max-block-size",
-    "160": "max-height",
-    "161": "max-inline-size",
-    "162": "max-width",
-    "163": "min-block-size",
-    "164": "min-height",
-    "165": "min-inline-size",
-    "166": "min-width",
-    "167": "mix-blend-mode",
-    "168": "object-fit",
-    "169": "object-position",
-    "170": "opacity",
-    "171": "order",
-    "172": "outline-color",
-    "173": "outline-offset",
-    "174": "outline-style",
-    "175": "outline-width",
-    "176": "overflow-x",
-    "177": "overflow-y",
-    "178": "padding-block-end",
-    "179": "padding-block-start",
-    "180": "padding-bottom",
-    "181": "padding-inline-end",
-    "182": "padding-inline-start",
-    "183": "padding-left",
-    "184": "padding-right",
-    "185": "padding-top",
-    "186": "position",
-    "187": "r",
-    "188": "right",
-    "189": "rotate",
-    "190": "row-gap",
-    "191": "rx",
-    "192": "ry",
-    "193": "scale",
-    "194": "scrollbar-gutter",
-    "195": "scrollbar-width",
-    "196": "stop-color",
-    "197": "stop-opacity",
-    "198": "table-layout",
-    "199": "text-decoration-color",
-    "200": "text-decoration-style",
-    "201": "text-decoration-thickness",
-    "202": "text-overflow",
-    "203": "top",
-    "204": "transform",
-    "205": "transform-box",
-    "206": "transform-origin",
-    "207": "transition-delay",
-    "208": "transition-duration",
-    "209": "transition-property",
-    "210": "transition-timing-function",
-    "211": "translate",
-    "212": "unicode-bidi",
-    "213": "user-select",
-    "214": "vertical-align",
-    "215": "view-transition-name",
-    "216": "width",
-    "217": "x",
-    "218": "y",
-    "219": "z-index"
+    "156": "mask-image",
+    "157": "mask-type",
+    "158": "max-block-size",
+    "159": "max-height",
+    "160": "max-inline-size",
+    "161": "max-width",
+    "162": "min-block-size",
+    "163": "min-height",
+    "164": "min-inline-size",
+    "165": "min-width",
+    "166": "mix-blend-mode",
+    "167": "object-fit",
+    "168": "object-position",
+    "169": "opacity",
+    "170": "order",
+    "171": "outline-color",
+    "172": "outline-offset",
+    "173": "outline-style",
+    "174": "outline-width",
+    "175": "overflow-x",
+    "176": "overflow-y",
+    "177": "padding-block-end",
+    "178": "padding-block-start",
+    "179": "padding-bottom",
+    "180": "padding-inline-end",
+    "181": "padding-inline-start",
+    "182": "padding-left",
+    "183": "padding-right",
+    "184": "padding-top",
+    "185": "position",
+    "186": "r",
+    "187": "right",
+    "188": "rotate",
+    "189": "row-gap",
+    "190": "rx",
+    "191": "ry",
+    "192": "scale",
+    "193": "scrollbar-gutter",
+    "194": "scrollbar-width",
+    "195": "stop-color",
+    "196": "stop-opacity",
+    "197": "table-layout",
+    "198": "text-decoration-color",
+    "199": "text-decoration-style",
+    "200": "text-decoration-thickness",
+    "201": "text-overflow",
+    "202": "top",
+    "203": "transform",
+    "204": "transform-box",
+    "205": "transform-origin",
+    "206": "transition-delay",
+    "207": "transition-duration",
+    "208": "transition-property",
+    "209": "transition-timing-function",
+    "210": "translate",
+    "211": "unicode-bidi",
+    "212": "user-select",
+    "213": "vertical-align",
+    "214": "view-transition-name",
+    "215": "width",
+    "216": "x",
+    "217": "y",
+    "218": "z-index"
 }
 All properties associated with document.body.style by default:
 {}

Tests/LibWeb/Text/expected/css/getComputedStyle-print-all.txt

@@ -154,7 +154,6 @@ margin-inline-start: 8px
 margin-left: 8px
 margin-right: 8px
 margin-top: 8px
-mask: none
 mask-image: none
 mask-type: luminance
 max-block-size: none