Add .editorconfig shared editor configuration

[EditorConfig] is a format for specifying configuration information,
such as indentation, supported by many common editors and IDEs.  A
benefit is encouraging consistency across editors from a single shared
configuration file, rather than editor-specific modelines in each file
or requiring each developer to configure their own project-specific
settings.

This commit adds an .editorconfig file with settings that match the
most common current indent styles.  It is not intended to proscribe any
changes to current style, but to encourage consistency going forward.

[EditorConfig]: https://editorconfig.org

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>
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