Add a vim modeline and .editorconfig

To make indentation work with less effort. The modeline was copied from
libostree with minor modification and the .editorconfig from GLib.

The advantage of having both a modeline and an editorconfig is we can
work out of the box on more editor setups, and the modeline allows us to
specify the style with a lot more fine grained control.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f