search notebooks: file block type

Created by: novoselrok

The new type is defined in client/web/src/search/notebook/index.ts as well as fetching the highlighted file.

To serialize a file block, I'm using the URL path format we use in the web app (e.g. /github.com/sourcegraph/sourcegraph/-/blob/internal/database/repos.go?L1007:12). The benefit is that we already have the functions to parse and construct the path and we do not have to invent a new format. We could also include the domain name, because notebooks aren't really expected to be transferrable between instances.

Fixes #26975 (closed)