sl-breadcrumb-item: Allow using sl-dropdown in the default slot

[schilchSICKAG]

As currently documented, sl-dropdown elements can be added to the prefix or suffix slot of a <sl-breadcrumb-item>. We also have the use-case of showing a dropdown as the sole item in a breadcrumb, as can be seen in this codepen. Basically, we want to show only the first and last entry, but hide everything other navigation trail item in a <sl-dropdown>.

This is currently not possible because you will get an automatic focus as <sl-breadcrumb-item> always renders an <a> or <button>. Also the main content has an automatic text-alignment of center, which breaks the styles of <sl-dropdown> / <sl-menu>.

I have created a quick branch for this feature and would propose to add this (or a better version if it if we can come up with one) to allow mounting <sl-dropdown> directly into the default slot.

[claviska]

@lindsaym-fa is this a use case we want to support for breadcrumbs?

[lindsaym-fa]

@claviska It's a valid use case, so I'm all for supporting it.

[schilchSICKAG]

I created a first PR at #2015 for this feature. Please tell me what you think :).