feat(overlay): Add onOpen and onSevereEvent callbacks

[Lms24]

This PR adds two public API callbacks:

• onOpen - fired when the debug window is opened
• onSevereEvent - fired when an integration processes and returns a severe event

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
spotlightjs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Nov 28, 2023 6:10pm

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (49d4491) 32.73% compared to head (1207bc9) 32.73%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #161   +/-   ##
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  Coverage   32.73%   32.73%           
=======================================
  Files          36       36           
  Lines        1720     1720           
  Branches       63       63           
=======================================
  Hits          563      563           
  Misses       1157     1157           

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