Auto merge of #25923 - iulianR:issue-22312, r=jdm

Add VTTRegion and part of VTTCue DOM interfaces

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Hello! In this PR I implemented the VTTRegion DOM interface and part of VTTCue (#22312). Before continuing I thought it's maybe a good idea to first request a review or even merge what I did up to this point, as I might get stuck on the next part. I have a slight idea of what needs to be done (I assume it's hooking the GStreamer parser somewhere in `GetCueAsHTML()`), but a short outline or some more instructions would help a lot.

I updated test expectations by first running:
```
./mach test-wpt tests/wpt/web-platform-tests/webvtt/api --log-raw servo.log --pref dom.webvtt.enabled=true
```
then
```
./mach update-wpt servo.log
```
Thanks!
---
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- [ ] These changes fix #___ (GitHub issue number if applicable)

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components/config/prefs.rs

@@ -294,6 +294,9 @@ mod gen {
                     event_polling_interval: i64,
                     test: bool,
                 },
+                webvtt: {
+                    enabled: bool,
+                },
                 webxr: {
                     #[serde(default)]
                     enabled: bool,