Add experimental screen_reader feature

Part of https://github.com/emilk/egui/issues/167

egui_web/src/screen_reader.rs

@@ -0,0 +1,47 @@
+pub struct ScreenReader {
+    #[cfg(feature = "screen_reader")]
+    tts: Option<tts::TTS>,
+}
+
+#[cfg(not(feature = "screen_reader"))]
+impl Default for ScreenReader {
+    fn default() -> Self {
+        Self {}
+    }
+}
+
+#[cfg(feature = "screen_reader")]
+impl Default for ScreenReader {
+    fn default() -> Self {
+        let tts = match tts::TTS::default() {
+            Ok(screen_reader) => {
+                crate::console_log("Initialized screen reader.");
+                Some(screen_reader)
+            }
+            Err(err) => {
+                crate::console_warn(format!("Failed to load screen reader: {}", err));
+                None
+            }
+        };
+        Self { tts }
+    }
+}
+
+impl ScreenReader {
+    #[cfg(not(feature = "screen_reader"))]
+    pub fn speak(&mut self, _text: &str) {}
+
+    #[cfg(feature = "screen_reader")]
+    pub fn speak(&mut self, text: &str) {
+        if text.is_empty() {
+            return;
+        }
+        if let Some(tts) = &mut self.tts {
+            crate::console_log(format!("Speaking: {:?}", text));
+            let interrupt = true;
+            if let Err(err) = tts.speak(text, interrupt) {
+                crate::console_warn(format!("Failed to read: {}", err));
+            }
+        }
+    }
+}