Include the menu loading promises in the default startup page ready promise chain. (mathjax/MathJax#3401)

[dpvc]

This PR adds the menu's loadingPromise into the promise chain for the initial typeset (and therefore the MathJax.startup.promise). This makes sure that everything is loaded before MathJax's initial typesetting occurs. This should only affect those loading components by hand rather than through a combined component.

Resolves issue mathjax/MathJax#3401.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.72%. Comparing base (3167719) to head (c6d089e).
⚠️ Report is 9 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1326   +/-   ##
========================================
  Coverage    86.72%   86.72%           
========================================
  Files          337      337           
  Lines        84131    84134    +3     
  Branches      3139     3140    +1     
========================================
+ Hits         72961    72964    +3     
  Misses       11170    11170           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[zorkow]

lgtm.