Change pointwise_logdensities default key type to VarName

[mhauru]

Closes #1070

[github-actions]

DynamicPPL.jl documentation for PR #1071 is available at:
https://TuringLang.github.io/DynamicPPL.jl/previews/PR1071/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.40%. Comparing base (01bf0bc) to head (9571f58).
⚠️ Report is 1 commits behind head on breaking.

Additional details and impacted files

@@             Coverage Diff              @@
##           breaking    #1071      +/-   ##
============================================
- Coverage     82.45%   82.40%   -0.06%     
============================================
  Files            42       42              
  Lines          3791     3791              
============================================
- Hits           3126     3124       -2     
- Misses          665      667       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[penelopeysm]

Thanks!