Allow disabling automatic backup files (with the ~ suffix)

Code follows the gedit tradition of saving backup~ files while editing (until the tab is closed).

Coming from macOS, this behavior feels a little alien to me, and it's redundant since my files are usually managed by git. It also clutters up my `git status` while I have Code open.

I propose an option to disable this feature altogether, or maybe it could be moved to a plugin that can be disabled.

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