Support multi-threaded Wasm (#3236)

Replace `atomic_refcell` with `parking_lot` on wasm32.

`parking_lot` has had problems running on wasm32 before
(https://github.com/emilk/egui/issues/1401)
but it works these days.
If we have problems again we can always switch to `std::sync::Mutex`.

Closes https://github.com/emilk/egui/issues/3102

Cargo.lock

@@ -326,12 +326,6 @@ dependencies = [
  "system-deps",
 ]
 
-[[package]]
-name = "atomic_refcell"
-version = "0.1.10"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "79d6dc922a2792b006573f60b2648076355daeae5ce9cb59507e5908c9625d31"
-
 [[package]]
 name = "atspi"
 version = "0.10.1"
@@ -1394,7 +1388,6 @@ version = "0.22.0"
 dependencies = [
  "ab_glyph",
  "ahash 0.8.3",
- "atomic_refcell",
  "backtrace",
  "bytemuck",
  "criterion",