feat(shorebird_cli): default to current app in shorebird preview

[bryanoltman]

Description

When running shorebird preview in a shorebird app project, default to that project.

Fixes #1115

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0b2e905) 100.00% compared to head (9c512ed) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1530   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          154       154           
  Lines         4761      4763    +2     
=========================================
+ Hits          4761      4763    +2     

Flag	Coverage Δ
shorebird_cli	100.00% <100.00%> (ø)

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