Guard the vendor event emission with obs_get_module("obs-websocket").
During obs_shutdown() modules are unloaded one-by-one; obs-websocket is
unloaded before advanced-scene-switcher, so the proc handler pointer (_ph)
cached in obs-websocket-api.h is already stale when FreeSceneSwitcher()
runs its stop steps. proc_handler_call() then dereferences freed memory
(use-after-free in pthread_mutex_unlock).
The existing OBSIsShuttingDown() guard does not catch this path: the
forced-exit that follows a "Source Cleanup Error" (scene collection
switch) never fires the SCRIPTING_SHUTDOWN frontend event, so the
plugin's obsIsShuttingDown flag is not set. obs_get_module() returns NULL
once the module is unloaded, which is the reliable signal that
obs-websocket's proc handler is gone.
Refs: #1404
Adds websocket vendor requests to start and stop the plugin.
Adds vendor events to indicate when the plugin is started or stopped.
These are ntended to be used for the StreamDeck plugin support.
