fix(runtime-dom): enable set form attr to null on form-elements (#2840)

[luwuer]

Close #2840

[LinusBorg]

Looking good, thanks!

Just a typo needing to be fixed, see comment.

[LinusBorg]

Typo need to be fixed, see comment.

Edit: commited the fix myself.

[LinusBorg]

This should be good now!

[LinusBorg]

I thought about this a bit more, and I think this solution is good for this individual issue, but there are more readonly properties out there that could fail like this.

Example: #2766

I think the core reason is that the last line of shouldSetAsProp only checks for the existance of the key, but not wether it has a setter:

https://github.com/vuejs/vue-next/blob/92d7ad18296fe43a01d10f38e03b983188dfba5a/packages/runtime-dom/src/patchProp.ts#L112

Instead of adding new exceptions for individual properties like we do here, we could do something like this:

return key in el && !!Object.getOwnPropertyDescriptor(el.__proto__, key)?.set

But I'm not 100% sure if thats a) performance and b) safe for all elements. It works for not the button.form and textarea.type scenarios.

Is there another (better?) way to check if a prop is readonly?

[posva]

We could still cache it for perf reasons but I wonder if there isn't an exception in a particular browser that makes the property readonly without setting the proper flag (you never know with JS!)

I don't find having a list of exception that bad unless it begins to get too big. It also allows for specific checks and better handling of edge cases

[LinusBorg]

agreed. I sent a seperate PR for the textarea thing. This one here is again good to go.

[yyx990803]

Removed tag check in 180310c

The form tag list adds to the baseline size and isn't really worth it.