feat(lockfile): add @generated comment

[BlueGlassBlock]

Pull Request Check List

• A news fragment is added in news/ describing what is new.
• Test cases added for changed code.

Describe what you have changed in this PR.

A comment containing @generated will be automatically added to the beginning of pdm.lock.

Resolves #1611.

[codecov-commenter]

Codecov Report

Base: 85.33% // Head: 85.26% // Decreases project coverage by -0.06% ⚠️

Coverage data is based on head (850fda5) compared to base (c4e1a4c).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1612      +/-   ##
==========================================
- Coverage   85.33%   85.26%   -0.07%     
==========================================
  Files          92       92              
  Lines        8480     8488       +8     
  Branches     1995     1828     -167     
==========================================
+ Hits         7236     7237       +1     
- Misses        839      843       +4     
- Partials      405      408       +3     

Flag	Coverage Δ
unittests	85.08% <100.00%> (-0.07%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/pdm/project/lockfile.py	100.00% <100.00%> (ø)
src/pdm/models/specifiers.py	77.54% <0.00%> (-1.76%)	⬇️
src/pdm/models/candidates.py	86.82% <0.00%> (-0.60%)	⬇️

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