file_contents_sorter: add --group-cases-together -- a better case-insensitive sort

[programmerjake]

The new --group-cases-together option groups lines that differ only in case together, so e.g.:

c
b
a
B

is sorted to:

a
B
b
c

instead of:

B
a
b
c

It also works with --unique.

The --ignore-case option keeps lines that differ only in case in the original sorted order, whereas the new option will sort them (and remove duplicates if you pass --unique).

I added a new option instead of changing --ignore-case for backward compatibility with existing commit hooks so it doesn't break CI.

better naming suggestions are welcome.

[asottile]

please discuss features first before spending time on them

I'm not really looking to add more options especially when I can't possibly think of a name for them or explain in simple terms how they work so I'm going to decline this sorry