Limit the meaning of "custom element" to not include is (#26524)

This PR has a bunch of surrounding refactoring. See individual commits.

The main change is that we no longer special case `typeof is ===
'string'` as a special case according to the
`enableCustomElementPropertySupport` flag.

Effectively this means that you can't use custom properties/events,
other than the ones React knows about on `<input is="my-input">`
extensions.

This is unfortunate but there's too many paths that are forked in
inconsistent ways since we fork based on tag name. I think __the
solution is to let all React elements set unknown properties/events in
the same way as this flag__ but that's a bigger change than this flag
implies.

Since `is` is not universally supported yet anyway, this doesn't seem
like a huge loss. Attributes still work.

We still support passing the `is` prop and turn that into the
appropriate createElement call.

@josepharhar

DiffTrain build for [43a70a6](43a70a6)

compiled/facebook-www/REVISION

@@ -1 +1 @@
-1a1d61fed98a02c9b1bac029d0bc11c3e4db896d
+43a70a610dd2cc298ab5592deebfbf8f7bbac127

compiled/facebook-www/React-dev.modern.js

@@ -27,7 +27,7 @@ if (
 }
           "use strict";
 
-var ReactVersion = "18.3.0-www-modern-8f5a57f2";
+var ReactVersion = "18.3.0-www-modern-b298fcd9";
 
 // ATTENTION
 // When adding new symbols to this file,

compiled/facebook-www/ReactART-dev.classic.js

@@ -69,7 +69,7 @@ function _assertThisInitialized(self) {
   return self;
 }
 
-var ReactVersion = "18.3.0-www-classic-26d7a5d5";
+var ReactVersion = "18.3.0-www-classic-8643ce3a";
 
 var LegacyRoot = 0;
 var ConcurrentRoot = 1;

compiled/facebook-www/ReactART-prod.modern.js

@@ -9688,7 +9688,7 @@ var slice = Array.prototype.slice,
       return null;
     },
     bundleType: 0,
-    version: "18.3.0-www-modern-132fb191",
+    version: "18.3.0-www-modern-b8a2f36f",
     rendererPackageName: "react-art"
   };
 var internals$jscomp$inline_1304 = {
@@ -9719,7 +9719,7 @@ var internals$jscomp$inline_1304 = {
   scheduleRoot: null,
   setRefreshHandler: null,
   getCurrentFiber: null,
-  reconcilerVersion: "18.3.0-www-modern-132fb191"
+  reconcilerVersion: "18.3.0-www-modern-b8a2f36f"
 };
 if ("undefined" !== typeof __REACT_DEVTOOLS_GLOBAL_HOOK__) {
   var hook$jscomp$inline_1305 = __REACT_DEVTOOLS_GLOBAL_HOOK__;