Implement varname prefix / unprefix

[penelopeysm]

Closes #117

This will eventually be needed for submodel stuff.

[codecov]

Codecov Report

Attention: Patch coverage is 84.31373% with 8 lines in your changes missing coverage. Please review.

Project coverage is 90.93%. Comparing base (700a70b) to head (b10bd1b).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/varname.jl	84.31%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #119      +/-   ##
==========================================
- Coverage   92.30%   90.93%   -1.37%     
==========================================
  Files           2        2              
  Lines         247      298      +51     
==========================================
+ Hits          228      271      +43     
- Misses         19       27       +8     

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[coveralls]

Pull Request Test Coverage Report for Build 13652741018

Details

• 40 of 51 (78.43%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.4%) to 90.94%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/varname.jl	40	51	78.43%

Totals
Change from base Build 13229542274:	-1.4%
Covered Lines:	271
Relevant Lines:	298

---

💛 - Coveralls

[sunxd3]

Looks great!

Only minor concern is that the tests seem to overlap with doctests, might still worth keeping, but I just thought to mention this

[penelopeysm]

It is, I did notice it too. :( Do you have any idea what's the best thing to do? Feels bad to remove the tests, but we do also want to document the behaviour.

[sunxd3]

From my end, I think it okay to keep both