ADD a STORY_UNCHANGED event

[tmeasday]

Emit when the STORY_RENDER event is received but nothing has changed, so we aren't re-rendering.

This helps tools like Chromatic detect with certainty when a story has finished rendering

[shilman]

YAY 🎉

[codecov]

Codecov Report

Merging #5340 into next will increase coverage by <.01%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##             next    #5340      +/-   ##
==========================================
+ Coverage   30.08%   30.08%   +<.01%     
==========================================
  Files         619      619              
  Lines        9001     9003       +2     
  Branches     1230     1248      +18     
==========================================
+ Hits         2708     2709       +1     
- Misses       5628     5629       +1     
  Partials      665      665

Impacted Files	Coverage Δ
lib/core/src/client/preview/start.js	76.25% <0%> (-0.97%)	⬇️
lib/core-events/src/index.ts	100% <100%> (ø)	⬆️
lib/theming/src/themes/dark-syntax.ts
lib/theming/src/themes/light-syntax.ts	0% <0%> (ø)

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