fix(css): Update the default/initial value of offset-position (#709)

### Description

This PR updates the initial value of
[`offset-position`](https://developer.mozilla.org/en-US/docs/Web/CSS/offset-position)
property from `auto` to `normal`.

**Sources:**

- Spec: https://drafts.fxtf.org/motion/#offset-position-property
- Change in spec from `auto` to `normal`:
w3c/fxtf-drafts@fc81ee1
- Change implemented in Firefox:
    - https://bugzilla.mozilla.org/show_bug.cgi?id=1835434
    - https://bugzilla.mozilla.org/show_bug.cgi?id=1559232

To test:
1. Open this example in Playground (Firefox 122 beta, releasing on Jan
23): [Comparing various offset starting
positions](https://developer.mozilla.org/en-US/docs/Web/CSS/offset-position#comparing_various_offset_starting_positions).
2. Unset the value of `offset-position` to see the default value
(default position with `normal` is (50%, 50%) of the containing block).

### Motivation

The examples shown in content are currently based on the
`offset-position`'s earlier default value `auto` and are not behaving as
expected. The default value impacts the behavior of other properties
like `ray()` in `offset-path` that depend on `offset-position`. I'll be
opening a corresponding content PR to adjust the examples to show the
updated/expected behavior.

css/properties.json

@@ -7271,7 +7271,7 @@
     "groups": [
       "CSS Motion Path"
     ],
-    "initial": "auto",
+    "initial": "normal",
     "appliesto": "transformableElements",
     "computed": "forLengthAbsoluteValueOtherwisePercentage",
     "order": "perGrammar",