Add .editorconfig

Editorconfig is a standard that helps editors know what whitespace rules
to follow for a given project.

For more information, see:

https://editorconfig.org/

These rules were determined based on feedback from project maintainers.

There are some files that use different numbers of spaces, but these
files were imposed on the project externally.  We use 4 everywhere we
touch.

Issue #2716

.editorconfig

+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+indent_size = 4
+indent_style = space
+insert_final_newline = true
+trim_trailing_whitespace = true