Add Linux desktop environment detection utility

Introduced `LinuxDesktopEnvironment` enum and `detectLinuxDesktopEnvironment()` function to identify Linux desktop environments using common environment variables like `XDG_CURRENT_DESKTOP` and `DESKTOP_SESSION`. Added fallback support for unrecognized environments.

Author: kdroidFilter

platformtools/core/src/jvmMain/kotlin/io/github/kdroidfilter/platformtools/LinuxEnvironment.kt

@@ -0,0 +1,31 @@
+package io.github.kdroidfilter.platformtools
+
+/** Most common Linux desktop environments. */
+enum class LinuxDesktopEnvironment {
+    GNOME, KDE, XFCE, CINNAMON, MATE, UNKNOWN
+}
+
+/**
+ * Detects the Linux Desktop Environment.
+ *
+ * - Returns **null** if the OS is not Linux († avoids unnecessary computation).
+ * - Uses common environment variables:
+ *   `XDG_CURRENT_DESKTOP`, `DESKTOP_SESSION`.
+ */
+fun detectLinuxDesktopEnvironment(): LinuxDesktopEnvironment? {
+    if (getOperatingSystem() != OperatingSystem.LINUX) return null
+
+    val combinedEnv = buildList {
+        System.getenv("XDG_CURRENT_DESKTOP")?.let(::add)
+        System.getenv("DESKTOP_SESSION")?.let(::add)
+    }.joinToString("|").lowercase()
+
+    return when {
+        "gnome"    in combinedEnv               -> LinuxDesktopEnvironment.GNOME
+        "kde" in combinedEnv || "plasma" in combinedEnv -> LinuxDesktopEnvironment.KDE
+        "xfce"     in combinedEnv               -> LinuxDesktopEnvironment.XFCE
+        "cinnamon" in combinedEnv               -> LinuxDesktopEnvironment.CINNAMON
+        "mate"     in combinedEnv               -> LinuxDesktopEnvironment.MATE
+        else                                    -> LinuxDesktopEnvironment.UNKNOWN
+    }
+}

platformtools/core/src/jvmMain/kotlin/io/github/kdroidfilter/platformtools/OsProvider.jvm.kt

@@ -8,4 +8,4 @@ actual fun getOperatingSystem(): OperatingSystem {
         osName.contains("nix") || osName.contains("nux") || osName.contains("aix") -> OperatingSystem.LINUX
         else -> OperatingSystem.UNKNOWN
     }
-}
+}