register method
Registers a plugin.
If a plugin with the same ZorphyPlugin.name is already registered, the new plugin replaces it.
Implementation
void register(ZorphyPlugin plugin) {
final idx = _plugins.indexWhere((p) => p.name == plugin.name);
if (idx >= 0) {
_plugins[idx] = plugin;
} else {
_plugins.add(plugin);
}
}