feat: add Slint file type definition

Co-Authored-By: Claude (auto) <noreply@anthropic.com>

src/main/kotlin/me/zhouxi/slint/lang/SlintFileType.kt

@@ -0,0 +1,15 @@
+package me.zhouxi.slint.lang
+
+import com.intellij.openapi.fileTypes.LanguageFileType
+import me.zhouxi.slint.icons.SlintIcons
+import javax.swing.Icon
+
+object SlintFileType : LanguageFileType(SlintLanguage) {
+    override fun getName(): String = "Slint"
+
+    override fun getDescription(): String = "Slint UI file"
+
+    override fun getDefaultExtension(): String = "slint"
+
+    override fun getIcon(): Icon = SlintIcons.Primary
+}