feat: add option to expand vars when exporting

[frostming]

Signed-off-by: Frost Ming me@frostming.com

Pull Request Checklist

• 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.

[codecov-commenter]

Codecov Report

Patch coverage: 87.50% and project coverage change: -0.01 ⚠️

Comparison is base (9393362) 85.22% compared to head (032fe52) 85.21%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1997      +/-   ##
==========================================
- Coverage   85.22%   85.21%   -0.01%     
==========================================
  Files          99       99              
  Lines        9352     9363      +11     
  Branches     2038     2041       +3     
==========================================
+ Hits         7970     7979       +9     
- Misses        938      939       +1     
- Partials      444      445       +1     

Flag	Coverage Δ
unittests	85.01% <87.50%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
src/pdm/models/backends.py	83.69% <81.81%> (-1.02%)	⬇️
src/pdm/cli/commands/export.py	95.00% <100.00%> (+0.12%)	⬆️
src/pdm/formats/requirements.py	79.05% <100.00%> (+0.43%)	⬆️

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