Allow multi-cursor 'smart' goto-file

[emilyyyylime]

The gf (goto_file) command has the ability to automatically detect paths or URLs around the cursor if no selection is active (the cursor's start and end are on the same character).
Personally, my intuition was that the only condition for this mechanism to activate was that the selection width is zero, so I was surprised that I couldn't make use where I had a list of files in source code that I multi-selected with C.

This patch allows for gf to intelligently detect file names even with multiple selections active, with the condition that they must all be a single character wide.

[the-mikedavis]

The current behavior is very specific since I think it can be surprising when a command has two pretty different behaviors. (Opening only what's selected vs. detecting the file / URL.) I'm not necessarily against making this work when you have multiple cursors though (as this PR does) - I think this could be quite useful if you have a list of links or if we had a way of adding selections with the mouse.

[emilyyyylime]

@the-mikedavis it's already possible to add selections with alt+click isn't it?

[the-mikedavis]

Ah yep alt+click already works. (I'm not a big mouse user 😅)