tidy check for vim and emacs modelines
See #10719.
It skips *.webidl files for now since I am not sure where they come from and if they should be edited in tree or not.
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This is a new attempt of #10586, after Simon Sapin's great cleanups in #10749 has landed. I have adjusted the changes to the new structure that was introduced, and also only done a few of the longhand ones. Will certainly continue on this as soon as we have a basic agreement that this style is reasonable.
