Allow currentColor in canvas context's shadowColor (#30754)

This allows unifying the parse_color function and method, aligns Servo
with other browsers, and obeys the HTML spec:
 - https://html.spec.whatwg.org/multipage/canvas.html#shadows
 - https://html.spec.whatwg.org/multipage/infrastructure.html#parsed-as-a-css-color-value

tests/wpt/meta/MANIFEST.json

@@ -439502,7 +439502,7 @@
         []
        ],
        "shadows.yaml": [
-        "366ac0b6072dfaa9de58abbda84525d9fe1806d1",
+        "953ab2c555d5ad9d46d52b6c74f09d344850982c",
         []
        ],
        "text.yaml": [
@@ -621476,6 +621476,27 @@
          {}
         ]
        ],
+       "2d.shadow.attributes.shadowColor.current.basic.html": [
+        "bfdc54d31b71e663bb2eef282c2fb4c9655af98f",
+        [
+         null,
+         {}
+        ]
+       ],
+       "2d.shadow.attributes.shadowColor.current.changed.html": [
+        "9eee2122d5efb1c56f0ad695b90e1d2df54f2c5c",
+        [
+         null,
+         {}
+        ]
+       ],
+       "2d.shadow.attributes.shadowColor.current.removed.html": [
+        "e1989cbdc80622ff1598adf0485377c7d28777b6",
+        [
+         null,
+         {}
+        ]
+       ],
        "2d.shadow.attributes.shadowColor.initial.html": [
         "f4d0d33d6d9af347af413fdfc1436034a4a3c919",
         [