vsx-registry: better styling for extensions-view search field

[vince-fugnitto]

Bug Description:

The vsx-registry search field should be better styled when the view display is limited (ex: search-in-workspace does it well).
The view does not display the search field properly, and the placeholder should have ellipsis:

For comparison, here is vscode:

Steps to Reproduce:

1. start the application, and open the extensions view.
2. resize the view.
3. notice that the search field is not responsive, content is cut off, and the placeholder does not have the ellipsis.