Generated tracking code should init window._paq and only use async #16190

refs https://github.com/matomo-org/matomo/issues/16189
fix https://github.com/matomo-org/matomo/issues/16055

@Findus23 can you have a look?

I'm still using var _paq to keep the tracking code short. As otherwise would need to prefix every _paq.push code but could also instead prefix every variable with _paq. Once merged will need to update the docs on the developer site to use same style. Tested it in IE6 and it worked for me.

refs https://github.com/matomo-org/matomo/pull/13674