libstdc++: Add .editorconfig files

These config files set default formatting behaviour for a large number
of common editors, see https://editorconfig.org

The root=true setting in libstdc++-v3/.editorconfig prevents looking in
parent directories for additional settings. If we add a .editorconfig at
the top-level we might want to use root=true there instead, and allow
libstdc++-v3/.editorconfig to inherit some some settings from there (and
only override things we want to do differently).

libstdc++-v3/ChangeLog:

	* .editorconfig: New file.
	* include/std/.editorconfig: New file.