Rename configuration file format filename extension access method.
As mentioned in RINCL-23, the name ConfigurationFileFormat.getFilenameExtensionSuffixes() may be unnecessarily confusing. We can consider "extension" to indicate the thing after the dot, and consider the dot to be the delimiter, not the extension itself. So a better name might be ConfigurationFileFormat.getFilenameExtensions(), making clear in the documentation that some of these extensions may contain a dot (e.g. xml.properties.