Multi-file snippet editing: focus in the new blob's editor

When clicking "Add another file" button in multi-file snippet, we add the new content form for the blob right where the currently-existing content form exists. This means that it looks like visually nothing really changes on the screen, but to get to the new field users need to scroll.

Screen_Recording_2020-09-18_at_10.25.53

Proposal

When adding a new blob, focus on its path field. Alternatively focus right in the blob content field. No matter the solution, in both cases, the window should be scrolled so that users would face both fields and not the "Add another file" button