Add editorconfig to help OSS contributors

[brandonpayton]

When switching between many different open source code bases, it can be cumbersome to constantly change editor settings in order to respect a given project's basic style preferences. EditorConfig is a popular format for representing those preferences so editors can automatically respect them.

Many editors support EditorConfig out of the box, and many others have EditorConfig plugins: https://editorconfig.org/#pre-installed

This commit adds an EditorConfig to the ModSecurity project. It is most likely not perfect because the chosen indentation varies a bit throughout the project, but hopefully it provides a good basis for most ModSecurity editing.