Add withString

[johnynek]

close #434

[codecov-commenter]

Codecov Report

Merging #436 (2fd6eb2) into main (d2980d3) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #436      +/-   ##
==========================================
- Coverage   95.14%   95.07%   -0.07%     
==========================================
  Files           9        9              
  Lines        1338     1361      +23     
  Branches      124      130       +6     
==========================================
+ Hits         1273     1294      +21     
- Misses         65       67       +2     

Impacted Files	Coverage Δ
core/shared/src/main/scala/cats/parse/Parser.scala	94.56% <100.00%> (+0.02%)	⬆️
...shared/src/main/scala/cats/parse/Accumulator.scala	97.56% <0.00%> (-2.44%)	⬇️

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[regadas]

looks great!