move varname leaves code to AbstractPPL

[penelopeysm]

This is a straight-up copy-paste of preexisting code in DynamicPPL. Curiously, these functions were completely untested in DynamicPPL. (edit: ok there are doctests)
I haven't added new tests here as I want this PR to just be moving code. Hence the coverage CI looks like rubbish (because it is). But we definitely should add some tests.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 38 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.81%. Comparing base (8dddcea) to head (f0d3337).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/varname_leaves.jl	0.00%	38 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #134      +/-   ##
==========================================
- Coverage   86.28%   78.81%   -7.47%     
==========================================
  Files           5        6       +1     
  Lines         401      439      +38     
==========================================
  Hits          346      346              
- Misses         55       93      +38     

☔ 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.

[coveralls]

Pull Request Test Coverage Report for Build 17349859436

Details

• 0 of 38 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-7.5%) to 78.815%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/varname_leaves.jl	0	38	0.0%

Totals
Change from base Build 17290910905:	-7.5%
Covered Lines:	346
Relevant Lines:	439

---

💛 - Coveralls

[github-actions]

AbstractPPL.jl documentation for PR #134 is available at:
https://TuringLang.github.io/AbstractPPL.jl/previews/PR134/