Make node-main init() wait for the startup promise to resolve

[dpvc]

This PR modifies the node-main file's init() method to wait for the MathJax startup promise to resolve rather than completing while MathJax may be doing more startup tasks. This means node applications that use require('mathjax').init() (or the equivalent for import) don't have to wait on MathJax.startup.promise themselves.

This simplifies some of the node examples I'm working on.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.71%. Comparing base (4d1e195) to head (515f1d1).

Additional details and impacted files

@@            Coverage Diff             @@
##           develop    #1309     +/-   ##
==========================================
  Coverage    86.71%   86.71%             
==========================================
  Files          337      337             
  Lines        83979    83980      +1     
  Branches      4750     3125   -1625     
==========================================
+ Hits         72826    72827      +1     
- Misses       11130    11153     +23     
+ Partials        23        0     -23     

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[zorkow]

lgtm.