Svelte: Add search context selector

[taiyab]

The React app has the context filter applied by default, and gives you a way of managing search contexts through it.

We need to add a context selector prefixed to the input field that allows you to select and manage your search contexts.

We did some design work for this during the Finder design project, you can see it here: https://www.figma.com/file/EPgl0qVbqGSumvf52brhda/Code-Search?type=design&node-id=2330%3A111136&mode=design&t=4ysAYkRPYfuDcOXx-1

I'll adapt that work to a clear set of designs to work off for getting this into the Svelte work.