feat: Add support for resolving scratchpads relative to the current week

[notheotherben]

This PR introduces the ability to run commands like gt scratch ~5 or gt scratch ^2 to resolve the scratchpad from (respectively) 5 weeks ago or 2 weeks ago. This closes #1295

[codecov]

Codecov Report

Attention: Patch coverage is 97.82609% with 1 line in your changes missing coverage. Please review.

Project coverage is 84.40%. Comparing base (ae8f76c) to head (98c759f).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/core/resolver.rs	97.56%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1296      +/-   ##
==========================================
+ Coverage   84.32%   84.40%   +0.07%     
==========================================
  Files          91       91              
  Lines        9213     9257      +44     
==========================================
+ Hits         7769     7813      +44     
  Misses       1444     1444              

Files with missing lines	Coverage Δ
src/commands/scratch.rs	98.82% <100.00%> (+0.01%)	⬆️
src/completion/completer.rs	94.59% <100.00%> (+0.15%)	⬆️
src/core/resolver.rs	83.66% <97.56%> (+1.82%)	⬆️