[PHP] Find a way to shutdown the WASM module and reclaim memory

[adamziel]

At the moment there is no way to shutdown the WASM runtime after it was started.

The following addition to the Emscripten module was insufficient to free the allocated memory:

/**
 * Expose a way to shut down the PHP runtime
 */
Module["exitRuntime"] = function () {
    exitRuntime();
    const empty = new ArrayBuffer([]);
    Module["HEAP8"].set(empty);
    Module["HEAP16"].set(empty);
    Module["HEAP32"].set(empty);
    Module["HEAPU8"].set(empty);
    Module["HEAPU16"].set(empty);
    Module["HEAPU32"].set(empty);
    Module["HEAPF32"].set(empty);
    Module["HEAPF64"].set(empty);
    
    updateGlobalBufferAndViews(empty);

    Module["HEAP8"] = HEAP8 = null;
    Module["HEAP16"] = HEAP16 = null;
    Module["HEAP32"] = HEAP32 = null;
    Module["HEAPU8"] = HEAPU8 = null;
    Module["HEAPU16"] = HEAPU16 = null;
    Module["HEAPU32"] = HEAPU32 = null;
    Module["HEAPF32"] = HEAPF32 = null;
    Module["HEAPF64"] = HEAPF64 = null;

    wasmTable = null;
    wasmMemory = null;
    wasmBinary = null;
    for (let key in Module["asm"]) {
        Module["asm"][key] = null
    }
    for (let key in Module) {
        if(key !== 'websocket') {
            Module[key] = null
        }
    }
}

Playground uses workers on every platform and then shuts down the entire worker thread so this issue has a very low priority for now.