Implement stub Clipboard in iced_web

We need to figure out browser permissions and use of unstable `web-sys`
APIs

web/src/clipboard.rs

@@ -0,0 +1,21 @@
+/// A buffer for short-term storage and transfer within and between
+/// applications.
+#[derive(Debug, Clone, Copy)]
+pub struct Clipboard;
+
+impl Clipboard {
+    /// Creates a new [`Clipboard`].
+    pub fn new() -> Self {
+        Self
+    }
+
+    /// Reads the current content of the [`Clipboard`] as text.
+    pub fn read(&self) -> Option<String> {
+        unimplemented! {}
+    }
+
+    /// Writes the given text contents to the [`Clipboard`].
+    pub fn write(&mut self, _contents: String) {
+        unimplemented! {}
+    }
+}