add DropAre, UIKitPlaceholder and UIKitForm#Custom now have nullable target

src/jsMain/kotlin/dev/inmo/jsuikit/elements/DropArea.kt

@@ -0,0 +1,24 @@
+package dev.inmo.jsuikit.elements
+
+import androidx.compose.runtime.Composable
+import dev.inmo.jsuikit.modifiers.*
+import dev.inmo.jsuikit.utils.Attrs
+import org.jetbrains.compose.web.attributes.InputType
+import org.jetbrains.compose.web.dom.*
+import org.w3c.dom.HTMLDivElement
+import org.w3c.dom.HTMLInputElement
+
+@Composable
+fun DropArea(
+    attrs: Attrs<HTMLDivElement> = Attrs.empty(),
+    inputAttrs: Attrs<HTMLInputElement> = Attrs.empty(),
+    contentBuilder: ContentBuilder<HTMLDivElement> = {}
+) = Div(
+    {
+        include(UIKitPlaceholder, UIKitForm.Custom())
+        attrs.builder(this)
+    }
+) {
+    Input(InputType.File, attrs = { inputAttrs.builder.invoke(this) })
+    contentBuilder(this)
+}