Add isPersistentBackground

[fregante]

Event pages and service workers can be unloaded at any time. There should be a detection to tell a persistent background page from an unloadable one.

Does isPersistentBackground make sense? isEventPage could work but I'd prefer a detection that lumps event pages and service workers.

[fregante]

const background = chrome.runtime.getManifest().background!;
const isPersistentBackgroundPage =
	!('service_worker' in background)
		&& background.persistent !== false;

[fregante]

Usable here

https://github.com/fregante/webext-inject-on-install/blob/95acba15cef51bf533fcdc0605e541c25a431a95/source/inject.ts#L47