Rename serializable to shadowrootserializable

See the conversation here:
  whatwg/html#10139

But the TL;DR is that the `<template>` attribute corresponding
to the declarative shadow root `serializable` attribute should
be named in a parallel way to other DSD-related attributes,
meaning it should start with `shadowroot*`.

Bug: 41490936
Change-Id: I40ea496c22147bac7dcdfb74cd7bf4a3abae8be9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5375416
Commit-Queue: Di Zhang <dizhangg@chromium.org>
Reviewed-by: Di Zhang <dizhangg@chromium.org>
Auto-Submit: Mason Freed <masonf@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1274374}

shadow-dom/declarative/declarative-shadow-dom-repeats.html

@@ -55,7 +55,7 @@
 </script>
 
 <div id=open2>
-  <template shadowrootmode=open shadowrootdelegatesfocus shadowrootclonable serializable>
+  <template shadowrootmode=open shadowrootdelegatesfocus shadowrootclonable shadowrootserializable>
     Open, delegates focus (not the default), clonable (not the default)
     serializable (not the default), named slot assignment (the default)
   </template>

shadow-dom/declarative/gethtml.tentative.html

@@ -36,7 +36,7 @@
       default: throw new Error(`Invalid serializable ${serializable}`);
     }
     const delegatesAttr = delegatesFocus ? ' shadowrootdelegatesfocus=""' : '';
-    const serializableAttr = expectedSerializable ? ' serializable=""' : '';
+    const serializableAttr = expectedSerializable ? ' shadowrootserializable=""' : '';
     const clonableAttr = clonable ? ' shadowrootclonable=""' : '';
 
     if (allowsShadowDom && declarativeShadowDom) {