Enable useHunkModeInStagingView user config by default

[stefanhaller]

Now that we have this improved hunk selection behavior added in #4684 (which some users call a game changer, and I agree 😄), I would like to enable it by default, because I find this to be the more useful mode than line mode in most cases. We added a user config for switching to hunk mode by default, but it is opt-in for now.

The reason why I'm hesitant to enable it by default is mentioned in #4685: there are still cases where it is necessary to switch to line mode (e.g. you added two methods to a class right next to each other, but you want to add them in different commits), and then it's very non-obvious how to do that. New users might not find out at all, and think that lazygit doesn't allow staging individual lines.

Some possible options for improving this:

• change navigation such that up/down switches to line mode, and jumping to the next hunk needs to be done with right-arrow (like in line mode). This could be annoying for people who are used to navigating hunks with up/down in hunk mode, and like it.
• show a popup when first entering staging, explaining that hunk mode is now the default, but that line mode is also available and how to switch to it. We'd remember in State.yml whether we have shown it already.

Any other ideas?