Add top-level .editorconfig file

Add a .editorconfig [1] file.  This helps configure editors
automatically with the right whitespace settings.  It will help me,
since I need to juggle with different whitespace settings for different
projects.   But I think it can also help newcomers get things right from
the start.

Some editors have native support for reading these files, while others
require a plug-in [2].  And if you don't want to use it, then this file
doesn't change anything to your life.

I added rules for the kinds of files I edit most often, but more can be
added later.  I assumed that the rules were the same for GDB and the
other projects, but if that's not the case, we can always put
.editorconfig files in project subdirectories to override settings.

[1] https://editorconfig.org/
[2] https://editorconfig.org/#download

Change-Id: Ifda136d13877fafcf0d137fec8501f6a34e1367b