Make Appender Type extensible from other modules and the user

[ZachHaber]

Appender was a discriminated type union, which unfortunately made it so that it wasn't able to be modified. Now you can update the Appenders interface with new values to enable modification.

declare module './log4js' {
  interface Appenders {
    StorageTestAppender: {
      type: 'storageTest';
      storageMedium: 'dvd' | 'usb' | 'hdd';
    };
  }
}

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Most of the changes in this PR are prettier being run, as I also set up the editorconfig to apply properly to typescript files as well.

The gitattributes was also missing typescript files.

[lamweili]

@ZachHaber Thanks! 👍