feat: introduce watch mode

[wellwelwel]

For now, the watch mode watches its own test paths (not the src, for example).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.39%. Comparing base (7b94a13) to head (7afcfa6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #393      +/-   ##
==========================================
+ Coverage   95.28%   95.39%   +0.10%     
==========================================
  Files          34       36       +2     
  Lines        2802     2996     +194     
  Branches      427      444      +17     
==========================================
+ Hits         2670     2858     +188     
  Misses        125      125              
- Partials        7       13       +6     

Flag	Coverage Δ
linux-parallel	95.06% <100.00%> (+0.16%)	⬆️
linux-parallel-options	95.36% <100.00%> (+0.17%)	⬆️
linux-sequential	?
linux-sequential-options	?
osx-parallel	?
osx-parallel-options	95.36% <100.00%> (+0.17%)	⬆️
osx-sequential	?
osx-sequential-options	?
windows-parallel	?
windows-parallel-options	?
windows-sequential	?
windows-sequential-options	?

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