Improve search vs direct entry affordances for Link UI search

[getdave]

The input in the Link UI is both a search box and/or a means to directly add a link (e.g. www.wordpress.org).

To encourage searching let's make the input look more visually aligned with the search box in the block inserter. However, if the user types in something that conforms to the heuristic for a URL then we should switch out the icon to be a keyboard icon. This is intended to provide an additional affordance to confirm to the user that they are currently directly adding a URL rather than searching.

[richtabor]

We explored aligning the search/input mechanism with the other search components throughout the editor. Not 100% sure this works, but wanted to add the exploration:

Relates to #50176.