Add extensions for handling UniFile name/file extensions

(cherry picked from commit 46aeab9a7ac6fd5c45d426e3a733d716dcb1d25f)

# Conflicts:
#	app/src/main/java/eu/kanade/tachiyomi/data/download/DownloadCache.kt

core/src/main/java/tachiyomi/core/storage/UniFileExtensions.kt

@@ -0,0 +1,9 @@
+package tachiyomi.core.storage
+
+import com.hippo.unifile.UniFile
+
+val UniFile.extension: String?
+    get() = name?.substringAfterLast('.')
+
+val UniFile.nameWithoutExtension: String?
+    get() = name?.substringBeforeLast('.')