Auto merge of #20740 - fabricedesre:mutation-observer-disconnect, r=jdm

Implement MutationObserver.disconnect()

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This implements https://dom.spec.whatwg.org/#dom-mutationobserver-disconnect
I added a `node_list` to the `MutationObserver` struct to keep track of the nodes involved in an observer because otherwise I could not find a way to unregister the observers on the node themselves without traversing the whole document tree. Let me know if there's something I missed.

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- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [x] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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tests/wpt/metadata/dom/nodes/Element-classlist.html.ini

@@ -1,6 +1,5 @@
 [Element-classlist.html]
   type: testharness
-  expected: TIMEOUT
   [classList.remove("a") with attribute value null (HTML node)]
     expected: FAIL
 

tests/wpt/metadata/dom/nodes/MutationObserver-disconnect.html.ini

@@ -1,8 +0,0 @@
-[MutationObserver-disconnect.html]
-  type: testharness
-  [subtree mutations]
-    expected: FAIL
-
-  [disconnect discarded some mutations]
-    expected: FAIL
-

tests/wpt/metadata/dom/nodes/MutationObserver-document.html.ini

@@ -1,7 +1,5 @@
 [MutationObserver-document.html]
   type: testharness
-  [setup test]
-    expected: FAIL
 
   [parser insertion mutations]
     expected: FAIL

tests/wpt/metadata/dom/nodes/MutationObserver-takeRecords.html.ini

@@ -1,2 +0,0 @@
-[MutationObserver-takeRecords.html]
-  type: testharness