Exclude URLs that already overlap with defaults (on first launch too)

[fregante]

trafficstars

Extension of:

• https://github.com/fregante/webext-dynamic-content-scripts/issues/12

The above issue was fixed with:

https://github.com/fregante/webext-dynamic-content-scripts/blob/1816d8a316b0d592df4502511e5bb1b7f6b302ee/index.ts#L56

But it didn't cover injectToExistingTabs:

https://github.com/fregante/webext-dynamic-content-scripts/blob/1816d8a316b0d592df4502511e5bb1b7f6b302ee/index.ts#L27-L32

This is particularly visible when all_urls is granted, so it always covers all tabs.