Add opt-out fragile-send-sync-non-atomic-wasm feature for wgpu (#5098)

Note this will break people depending on eframe or egui-wgpu with
--no-default-features.
I don't know what to do about that to be honest.

* Closes #4914 
* [x] I have followed the instructions in the PR template

---------

Co-authored-by: Andreas Reich <r_andreas2@web.de>

crates/eframe/Cargo.toml

@@ -39,6 +39,7 @@ default = [
   "web_screen_reader",
   "winit/default",
   "x11",
+  "egui-wgpu?/fragile-send-sync-non-atomic-wasm",
 ]
 
 ## Enable platform accessibility API implementations through [AccessKit](https://accesskit.dev/).
@@ -185,7 +186,11 @@ objc2-app-kit = { version = "0.2.0", features = [
 # windows:
 [target.'cfg(any(target_os = "windows"))'.dependencies]
 winapi = { version = "0.3.9", features = ["winuser"] }
-windows-sys = { workspace = true, features = ["Win32_Foundation", "Win32_UI_Shell", "Win32_System_Com"] }
+windows-sys = { workspace = true, features = [
+  "Win32_Foundation",
+  "Win32_UI_Shell",
+  "Win32_System_Com",
+] }
 
 # -------------------------------------------
 # web: