Add check for BG offset consistency

[hongaaronc]

Description

In taiko, it's common to adjust the vertical postion of the background by editing values in the .osu file, due to the unique positioning of the playfield.

Usually if this value is not consistent given the same file name, it's probably a blunder. Currently this is something I have been checking manually, but I'd like to introduce it to Taiko MV as an automated check so I no longer have to spend time on that.

Emitting it as a minor issue, since sometimes it actually is intentional. I also put it as an all-modes check, since while it probably only would occur in Taiko, it would most likely be unintended if it happened in other modes as well.

Also, added Checks/Private to the .gitignore file to streamline development - I use this folder on my workspace for WIP checks, as well as checks only intended for debugging (such as checks to emit logs for the SR calc work).

Testing

Tested on this map which triggers the check.

Also tested on a map with multiple BGs to make sure each BG is handled separately.

[hongaaronc]

Changed it to show the diff names in parentheses after each offset group.

So for this map now it looks like this:

And looks like this if I adjust the Oni to match the Muzu offset:

I wasn't able to figure out the new line character thing btw, I tried '\n', @"<br>", @"<br/>", @"<\ br>", and sb.AppendLine() and none of them worked. Pretty sure the field on the webapp side just doesn't support it.