Add a .editorconfig file?

[Magnus Ihse Bursie]

As suggested in 
https://github.com/openjdk/jdk/pull/11491#issuecomment-1385301564, maybe 
we should add a .editorconfig file to the JDK project, to help IDEs and 
editors comply with jcheck rules.

To get this to work, I think we need to first convert the current 
jchecks rules into a matching .editorconfig file, and then verify that 
this works and has no unintended consequences on the major IDEs.

/Magnus