[button] Consider removing internal stylable parts

[jouni]

The stylable parts (prefix, suffix, and label) I introduced a while ago don’t really make much sense semantically. I’m considering we should remove them.

That said, removing them means that themes can’t target them, and that in turn makes it difficult to make baseline alignment work like it does currently, because we would only be able ot set the font-size and line-height on the host (which affects the baseline alignment).

[web-padawan]

We have discussed this and agreed about the following:

1. Let's remove spans and prefix & suffix parts (but keep the corresponding slots)
2. Let's not abuse slot elements for styling and instead style slotted content directly
3. We can use selectors like slot[name='prefix'] ::slotted(*) to apply margins etc
4. Users will be able to override these styles because the content is in the light DOM
5. Note, we also remove label part (already removed in the new implementation)
6. As a result, there will be only button part exposed by vaadin-button component.