egui_web: Add simple fetch API and demostrate it in example_web

egui_web/Cargo.toml

@@ -35,6 +35,7 @@ features = [
   'Document',
   'DomRect',
   'Element',
+  'Headers',
   'HtmlCanvasElement',
   'HtmlElement',
   'KeyboardEvent',
@@ -42,6 +43,10 @@ features = [
   'MouseEvent',
   'Navigator',
   'Performance',
+  'Request',
+  'RequestInit',
+  'RequestMode',
+  'Response',
   'Storage',
   'Touch',
   'TouchEvent',