MWPW-159197 Invoke onReady of standalone gnav when it is rendered

[sharmrj]

• We now do await gnav.init() so that onReady is called only when gnav.init is finished executing

Resolves: MWPW-159197

Test URLs:

• Before: https://main--milo--adobecom.hlx.page/?martech=off
• After: https://await-gnav-init--milo--sharmrj.hlx.page/?martech=off

[aem-code-sync]

	Page	Scores	Audits	Google
📱	/?martech=off
🖥️	/?martech=off

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.30%. Comparing base (501a373) to head (b436b7f).

Additional details and impacted files

@@           Coverage Diff           @@
##            stage    #2962   +/-   ##
=======================================
  Coverage   96.30%   96.30%           
=======================================
  Files         242      242           
  Lines       54873    54873           
=======================================
  Hits        52845    52845           
  Misses       2028     2028           

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[sharmrj]

In order to QA this PR I'll have to raise it from a branch off the main repo (as opposed to my fork)

[sharmrj]

See #2963