Add new command pytask dag to visualize the DAG.

[tobiasraabe]

Changes

Closes #77.

• Document the CLI and programmatic interface.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 18 lines in your changes missing coverage. Please review.

Project coverage is 90.05%. Comparing base (f17f499) to head (6dee619).
Report is 435 commits behind head on main.

Files	Patch %	Lines
src/_pytask/graph.py	85.43%	15 Missing ⚠️
tests/test_graph.py	93.75%	2 Missing ⚠️
src/_pytask/build.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #101      +/-   ##
==========================================
- Coverage   90.12%   90.05%   -0.07%     
==========================================
  Files          60       62       +2     
  Lines        5132     5270     +138     
==========================================
+ Hits         4625     4746     +121     
- Misses        507      524      +17     

Flag	Coverage Δ
end_to_end	76.54% <45.83%> (-0.87%)	⬇️
integration	41.53% <43.75%> (+0.03%)	⬆️
unit	67.26% <86.80%> (+0.89%)	⬆️

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