Allow specifying a set of lines in the URL to be highlighted in the UI

### Feature request

It's useful for emphasizing some parts of the code especially when people are asking for help. Similarly to how you can highlight a set of line in a URL via GitHub, like https://github.com/haskell/play-haskell/blob/master/hie.yaml#L9-L15

The UI for highlighting a URL could be similar to GitHub's, where it's different from text selection, and to highlight a single line you press on the line number on the left, and to highlight multiple lines you click on a line first then shift+click on a line to highlight the range of lines.

The issue names no files, tests, or entry points. Start by locating the URL handling and code-view UI, then compare the existing behavior with GitHub's line highlighting; done means line ranges can be encoded in URLs and selected through line-number clicks and shift-clicks.

Written by the indexing model from the issue text.