Bug: history should work with custom TextNodes containing non-standard properties

[CanRau]

I created my own TextNode to have random css class support via the added property __className and some methods like addClass. While trying it out via a button in my toolbar I realized that the editor wouldn't allow me to undo this change, while all other changes work as expected, checked lexical source code, playground code, issues and searched discord until I asked directly on Discord and after poking around a little @etrepum discovered the hardcoded isTextNodeUnchanged helper function which won't consider custom properties.

I think it would be nice to move isTextNodeUnchanged as a method maybe isUnchanged (or maybe isChanged or hasChanged or hasChanges) into TextNode so subclasses can override it when they need to support custom properties. Then the history plugin would call that method here instead of the hardcoded function.

This method might even live in LexicalNode so other nodes could also make use of it, tho I don't have a direct need for this right now I think.

Lexical version: 0.16.1

Steps To Reproduce

1. Create a CustomTextNode extending TextNode
2. Add a non-standard property like __className
3. Duplicate a method like setFormat from TextNode to set your property from 2
4. trigger this method somehow
5. Realize that undo won't work 😅

My addClass method

addClass(className: string): this {
	const self = this.getWritable();
	const classes = self.__className?.split(" ").map((s) => s.trim());
	classes.push(className);
	self.__className = classes.join(" ");
	return self;
}

The button onClick that calls it (my actual code is a little longer and inspired by $patchStyleText, tho this is enough to reproduce the problem)

onClick={() => {
    activeEditor.update(() => {
        const selection = $getSelection();
        if ($isRangeSelection(selection)) {
            const selectedNodes = selection.getNodes();
            if ($isTextNode(selectedNodes[0])) {
                selectedNodes[0].addClass("text-red-500");
            }
        }
    });
}}

[etrepum]

Adding some more context here, this hack came from #2258 where an older version of the playground's MaxLengthPlugin did not use $restoreEditorState (added in #2273). It may actually be possible to remove the hack entirely.