Support multi-threaded Wasm

[emilk]

Replace atomic_refcell with parking_lot on wasm32.

parking_lot has had problems running on wasm32 before (#1401) but it works these days.
If we have problems again we can always switch to std::sync::Mutex.

Closes #3102

[juancampa]

@emilk I just got a panic with parking_lot in wasm (Electron). It happened when calling animate_bool_with_time. I haven't investigated what to do yet but I figured I post it here in case you or someone else knows how to fix it or work around it.

Update: text version of callstack:

gaze.js:1069 panicked at 'Parking not supported on this platform', /Users/juan/.cargo/registry/src/index.crates.io-6f17d22bba15001f/parking_lot_core-0.9.7/src/thread_parker/wasm.rs:26:9

Stack:

Error
    at imports.wbg.__wbg_new_e7fbaa407e13d590 (https://file+.vscode-resource.vscode-cdn.net/Users/juan/git/membrane-vscode/vscode-extension/static/gaze/gaze.js:1075:21)
    at eframe::web::panic_handler::PanicSummary::new::hdfdd14a6fe521f74 (wasm://wasm/01ad0952:wasm-function[4306]:0x36ba7e)
    at eframe::web::panic_handler::PanicHandler::install::{{closure}}::h88d3643bc21aec77 (wasm://wasm/01ad0952:wasm-function[2064]:0x2a346e)
    at std::panicking::rust_panic_with_hook::h7d9d49945f5b5498 (wasm://wasm/01ad0952:wasm-function[4253]:0x368e65)
    at std::panicking::begin_panic::{{closure}}::h5a799e48bb920b1c (wasm://wasm/01ad0952:wasm-function[5985]:0x39e0e4)
    at std::sys_common::backtrace::__rust_end_short_backtrace::hc40c0445c905d574 (wasm://wasm/01ad0952:wasm-function[7137]:0x3a40bc)
    at std::panicking::begin_panic::h4b0cd84d8ff65ef6 (wasm://wasm/01ad0952:wasm-function[6042]:0x39ea5f)
    at <parking_lot_core::thread_parker::imp::ThreadParker as parking_lot_core::thread_parker::ThreadParkerT>::prepare_park::hb42c842297d9d702 (wasm://wasm/01ad0952:wasm-function[6741]:0x3a2dbf)
    at parking_lot::raw_rwlock::RawRwLock::lock_exclusive_slow::hef2a6792406c1d36 (wasm://wasm/01ad0952:wasm-function[972]:0x1c0d58)
    at egui::context::Context::animate_bool_with_time::h8218597c319b3b91 (wasm://wasm/01ad0952:wasm-function[3091]:0x318217)
    ```

[juancampa]

Nevermind!

This panic happened when the code was about to deadlock. Maybe this is how deadlock detection manifests in WASM right now?