Improve the Top Area as an extension point

[jtpio]

Problem

In #11443 we add a spacer to the top area to align items to the top right corner of the page.

Some extensions already add a spacer to the top bar to align the items to the right. Which means there could be multiple spacers if the user menu and the other extensions are activated on the page.

It might be fine for now, but maybe we should check how to make the top area more well defined so extensions can more easily add items there without stepping on each others toes.

Proposed Solution

We could just add that spacer independently of these plugins, by default in JupyterLab, and document using a rank < 900 to place items to the left, and rank > 900 to place them to the right.

Another approach would be to make the space to the right of the menu in the top bar a Toolbar like extension point. Which is what jupyterlab-topbar started experimented with.

Additional context

Discussed in #11443