feat: add --only-keep option to sync command

[frostming]

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.

[codecov-commenter]

Codecov Report

Merging #1191 (75ef5cf) into main (77528ff) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1191      +/-   ##
==========================================
- Coverage   85.46%   85.45%   -0.01%     
==========================================
  Files          78       78              
  Lines        6831     6834       +3     
  Branches     1564     1565       +1     
==========================================
+ Hits         5838     5840       +2     
  Misses        660      660              
- Partials      333      334       +1     

Flag	Coverage Δ
unittests	85.22% <100.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
pdm/cli/commands/sync.py	100.00% <ø> (ø)
pdm/cli/options.py	91.26% <ø> (ø)
pdm/cli/actions.py	79.11% <100.00%> (ø)
pdm/installers/synchronizers.py	86.38% <100.00%> (+0.11%)	⬆️
pdm/project/core.py	89.32% <0.00%> (-0.24%)	⬇️
pdm/project/config.py	94.11% <0.00%> (ø)

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[Hnasar]

I found the behavior of pure to be a bit hard to understand. A name like --only or --only-keep would be more intuitive. For example:

pdm sync --only-keep --prod

Some examples in the docs might help too.

[LukeDRussell]

Keen to see this in a pre-release tag.
I've been trying to migrate a small project to PDM. I've had to delete the venv every time to acheive a strict match to the 'working set' of the intended groups.