function getLanguageForExtension(extension) {
        return _fileExtensionToLanguageMap[extension.toLowerCase()];
    }
Resolves a file extension to a Language object.
*Warning:* it is almost always better to use getLanguageForPath(), since Language can depend
on file name and even full path. Use this API only if no relevant file/path exists.
@param {!string} extension Extension that language should be resolved for
@return {?Language} The language for the provided extension or null if none exists