Add container.ready event

[bingenito]

• Default base implementation simply returns a resolved promise
• OpenFin ready maps to fin.desktop.main()

Fixes #236

[bingenito]

@ggeorgievx-Tick42 How does this look at a core level?

[ggeorgievx-Tick42]

@bingenito Looks good to me, Brian. Thanks for the fast update 👌🏻🎉

[codecov]

Codecov Report

Merging #239 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #239      +/-   ##
=========================================
+ Coverage   93.38%   93.4%   +0.01%     
=========================================
  Files          16      16              
  Lines        1481    1485       +4     
  Branches      249     249              
=========================================
+ Hits         1383    1387       +4     
  Misses         98      98

Impacted Files	Coverage Δ
packages/desktopjs/src/container.ts	93.93% <100%> (+0.09%)	⬆️
packages/desktopjs-openfin/src/openfin.ts	86.84% <100%> (+0.07%)	⬆️