LibWeb/CSS: Add -moz-appearance property alias

Author: https://github.com/AtkinsSJ
Commit: https://github.com/LadybirdBrowser/ladybird/commit/cdc81e8921b
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/6821
Reviewed-by: https://github.com/tcl3

Libraries/LibWeb/CSS/Properties.json

@@ -1,4 +1,7 @@
 {
+  "-moz-appearance": {
+    "legacy-alias-for": "appearance"
+  },
   "-webkit-align-content": {
     "legacy-alias-for": "align-content"
   },

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

@@ -1,5 +1,7 @@
 All supported properties and their default values exposed from CSSStyleProperties from getComputedStyle:
 'cssFloat': 'none'
+'MozAppearance': 'none'
+'-moz-appearance': 'none'
 'WebkitAlignContent': 'normal'
 'webkitAlignContent': 'normal'
 '-webkit-align-content': 'normal'