You can already do this. Just triple-click to the immediate left of the line you want to select. To extend to a larger line block, with the first block still highlighted, shift-click on the line you want to delimit the other boundary. That line and all lines between the initially-selected line will then also be selected.
Postscriptum at a later date: it occurs to me that I didn’t mention a minor filip to this method. Once you’ve selected a line in line-block mode by triple-clicking, as I suggested (and, come to think of it, I don’t believe it even has to be to the left of the line; old habit from using Microsoft Word a lot), you can also simply drag to extend the line block.