feat(css): alsoAppliesTo data for bg-pos-x/bg-pos-y (#707)

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### Description

Copy the `alsoAppliesTo` data from `background-position` to
`background-position-x` and `background-position-y`, an obvious
derivation from the shorthand property.

### Motivation

Provide more complete/detailed "Formal definition" data for MDN CSS
property reference.

### Additional details

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places providing more context -->

### Related issues and pull requests

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css/properties.json

@@ -2329,6 +2329,11 @@
     "appliesto": "allElements",
     "computed": "listEachItemConsistingOfAbsoluteLengthPercentageAndOrigin",
     "order": "uniqueOrder",
+    "alsoAppliesTo": [
+      "::first-letter",
+      "::first-line",
+      "::placeholder"
+    ],
     "status": "experimental",
     "mdn_url": "https://developer.mozilla.org/docs/Web/CSS/background-position-x"
   },
@@ -2345,6 +2350,11 @@
     "appliesto": "allElements",
     "computed": "listEachItemConsistingOfAbsoluteLengthPercentageAndOrigin",
     "order": "uniqueOrder",
+    "alsoAppliesTo": [
+      "::first-letter",
+      "::first-line",
+      "::placeholder"
+    ],
     "status": "experimental",
     "mdn_url": "https://developer.mozilla.org/docs/Web/CSS/background-position-y"
   },