examples: add wasm online demo

examples/jotdown_wasm/src/lib.rs

@@ -0,0 +1,10 @@
+use wasm_bindgen::prelude::*;
+
+#[must_use]
+#[wasm_bindgen]
+pub fn jotdown_render(djot: &str) -> String {
+    let events = jotdown::Parser::new(djot);
+    let mut html = String::new();
+    jotdown::html::push(events, &mut html);
+    html
+}