Add mean argument to pre-specified laplace likelihoods

[WardBrian]

Counterpart to stan-dev/math#3210. Similarly, it is branched off of #1521 to avoid conflicts

Submission Checklist

• Run unit tests
• Documentation
  • If a user-facing facing change was made, the documentation PR is here:

Release notes

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[codecov]

Codecov Report

Attention: Patch coverage is 64.51613% with 11 lines in your changes missing coverage. Please review.

Project coverage is 89.72%. Comparing base (cc09d41) to head (ac187db).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/frontend/Semantic_error.ml	59.25%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1523      +/-   ##
==========================================
- Coverage   89.81%   89.72%   -0.10%     
==========================================
  Files          66       66              
  Lines        9887     9904      +17     
==========================================
+ Hits         8880     8886       +6     
- Misses       1007     1018      +11     

Files with missing lines	Coverage Δ
src/frontend/Typechecker.ml	93.43% <100.00%> (+0.07%)	⬆️
src/stan_math_signatures/Stan_math_signatures.ml	86.13% <ø> (ø)
src/frontend/Semantic_error.ml	91.30% <59.25%> (-4.03%)	⬇️

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