Source context UI - allow loading more lines/scrolling

### Problem Statement

When trying to fix an error with the help of a source code context, I often need to see more than just a couple of lines before the error line.

Because this info is not shown on the UI, I need to open the file at that specific revision and look for the code manually. This can get time consuming even for sources that are in the repositories I have checked out locally. For remote sources fetched for depdencies, this can get prohibitively complicated to do manually.

### Solution Brainstorm

Because Sentry already has access to the whole source file, it would be preferable to allow scrolling the file directly in the UI, to see the code leading to the frame line.

### Product Area

Issues