Allow threading for sync use_effect calls

[Archmonger]

Current Situation

Users frequently run into unexpected webserver "freezing" when using sync effects due to Python's single-threaded nature. This behavior doesn't seem to match up with equivalent JavaScript server side frameworks.

Proposed Actions

By default we should run all sync effects in a ThreadPoolExecutor(max_workers=1). This effectively runs all sync effects in a background thread. Additionally, by running all effects in the same worker thread (max_workers=1) this implementation is semi-threadsafe. For operations that must run on the main thread, we need a @use_effect(threading: bool = ...) parameter to switch back to legacy behavior.

We could also consider a reactpy.config.USE_EFFECT_WORKERS=... setting to allow users to modify the value of max_workers.