Allow <slot> to find slottables by slottables's IDs as well

[Zhang-Junzhi]

Sometimes, slottables already have IDs, so it's not necessary to require them to also define reduntant slot attributes. If a slot is able to find its slottables by slottables's IDs, we would go a step further to make the document semantically brief(It's semantical brevity which is one principial cause that inspires shadow DOM, isn't it?).

I'd like suggest we add a for attribute to for this.

<slot for="id-for-the-slottable"></slot>

[annevk]

A similar argument was made for element names, and no agreement was reached on that. An API so a library could implement this kind of functionality is likely the first thing implementers will find agreement on.

[annevk]

Such an API is whatwg/html#3534 btw.

[annevk]

F2F feedback: implementers don't want this. Per that, I'm closing this. Happy to discuss further if you have concerns though.