feat: add trim_final_newlines option

[yuhr]

Added an option to configure whether to trim final newlines.

I'm new to contribute to rustfmt, so I couldn't reasonably figure this to be stable or not (although I think it's a trivial option that there should never be a room for any kind of unstability).

[alex-semenyuk]

When this option can be helpful? I mean when it makes sense to have many newlines or not have at all.

[yuhr]

1. In my opinion, newlines at the EOF are essentially meaningless.
2. Editing EOF newlines may cause conflicts with other tooling such as Remove Final Newlines.

So, at least it should be configurable of whether to perform trimming. Even editorconfig is configurable of this.

It's weird nowadays that most programmers (and programs) force people to blindly conform to the ancient convention of POSIX which doesn't reflect human's natural intuition about what lines are. Humans are not slaves to machines.

Also see my comment on a similar issue in prettier/prettier:

From human's point of view, a newline character appears as rather the beginning of a newline, than merely the end of the line like POSIX defines. Many people think there's a pointless empty newline, probably that's one of the reason why they (and me of course) ended up into this issue. Softwares should be implemented as a reflection of how people think.