Preferring EditorConfig over modelines

[Gerald Combs <gerald () wireshark org>]

I updated the Developer's Guide and README.developer to recommend enabling EditorConfig in your editor or IDE with an eye toward 
getting rid of "Editor modelines" blocks as discussed in issue 17253[1]. They both do the same job, but EditorConfig is supported 
by more editors/IDEs, is more of a standard, and doesn't require adding extra stuff to each individual source file.

I don't plan on stripping out all of the modeline blocks any time soon, but I might remove them from ui/qt as an experiment. Qt 
Creator doesn't support modelines, so they're kind of superfluous there.

Note that I'm carefully avoiding the questions "Should we use the same indentation everywhwere?" and "Should we enforce a 
particular coding style, e.g. by using clang-format?", but we can discuss those too.

[1]https://gitlab.com/wireshark/wireshark/-/issues/17253
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