Update icon to include dark variant

CHANGELOG.md

@@ -8,6 +8,8 @@
 
 ### Changed
 
+- Icon of `*.nix` files is now using SVG format
+- Icon of `*.nix` files got a separate variant for dark backgrounds
 - Update Grammar-Kit: 2021.1.2 -> 2022.3.2
 - Update JetBrains' fork of JFlex: 1.7.0-1 -> 1.9.2
 

NOTICE

@@ -16,4 +16,5 @@ License Notice
 
    src/main/resources/META-INF/pluginIcon.svg
    src/main/resources/META-INF/pluginIcon_dark.svg
-   src/main/resources/icons/nixos-icon.png
+   src/main/resources/icons/nixSnowflake.svg
+   src/main/resources/icons/nixSnowflake_dark.svg

src/main/java/org/nixos/idea/icon/NixIcons.java

@@ -5,5 +5,12 @@
 import javax.swing.*;
 
 public class NixIcons {
-    public static final Icon FILE = IconLoader.getIcon("/icons/nixos-icon.png", NixIcons.class);
+
+    // Documentation
+    // -> https://plugins.jetbrains.com/docs/intellij/icons.html
+    // Design Guide
+    // -> https://jetbrains.design/intellij/principles/icons/
+
+    private static final Icon SNOWFLAKE = IconLoader.getIcon("/icons/nixSnowflake.svg", NixIcons.class);
+    public static final Icon FILE = SNOWFLAKE;
 }