feat(gui): add FlatLaf

Author: ngmikeng

README.md

@@ -578,6 +578,7 @@ _Libraries for working with geospatial data and algorithms._
 
 _Libraries to create modern graphical user interfaces._
 
+- [FlatLaf](https://github.com/JFormDesigner/FlatLaf) - Modern Swing Look and Feel with Darcula and IntelliJ themes.
 - [JavaFX](https://wiki.openjdk.java.net/display/OpenJFX/Main) - Successor of Swing.
 - [Scene Builder](https://gluonhq.com/products/scene-builder/) - Visual layout tool for JavaFX applications.
 - [SnapKit](https://github.com/reportmill/SnapKit) - Modern Java UI library for both desktop and web.