Navigation: Make Search block look nice when added to a Navigation

[noisysocks]

Here's how the Search block looks when added to a Navigation:

And here's the frontend:

When added to a Navigation, we should give Search a simpler design treatment that flows nicely with the rest of the links.

[draganescu]

Related #22071

[aristath]

Isn't this something that a theme should do?

[noisysocks]

Isn't this something that a theme should do?

Blocks come with base styles that a theme can then add to. We should ensure that the base styles are a good place for a theme to start from.

[shaunandrews]

The Search block is getting some customization options in #24666 — I'm not sure there's much the Navigation block needs to do here other than (maybe) toggle the Search label off when added to a horizontal Navigation block.