feat: add onScreenChanged, an app-level visible-screen signal

[Mastersam07]

closes #66

[github-actions]

📖 Docs preview: https://kaisel.dev/pr-77/ (updates on every push; removed when this PR closes)

[codecov]

Codecov Report

❌ Patch coverage is 86.53846% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.66%. Comparing base (8870f04) to head (72c93c9).
⚠️ Report is 3 commits behind head on dev.

Files with missing lines	Patch %	Lines
packages/kaisel/lib/src/kaisel_screen_signal.dart	84.00%	4 Missing ⚠️
...ackages/kaisel/lib/src/kaisel_inner_navigator.dart	70.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #77      +/-   ##
==========================================
- Coverage   99.95%   99.66%   -0.30%     
==========================================
  Files          42       43       +1     
  Lines        2332     2380      +48     
==========================================
+ Hits         2331     2372      +41     
- Misses          1        8       +7     

Flag	Coverage Δ
kaisel	99.66% <86.53%> (-0.30%)	⬇️
kaisel_core	99.66% <86.53%> (-0.30%)	⬇️
kaisel_devtools	99.66% <86.53%> (-0.30%)	⬇️
kaisel_lint	99.66% <86.53%> (-0.30%)	⬇️

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