Add Parser.caret

[johnynek]

This does the caret part of #238

related to #234

I did not add Span, just Caret.

My concern was that the span methods are a bit less clear. For instance, we may want an inclusive upper bound range, or maybe we do want exclusive. Inclusive is a bit more complex to implement however.

To sidestep this concern for now while we think about the design, I just added getting the Caret itself.

cc @re-xyr

[johnynek]

actually, inclusive span could be bad because that means you can never use it with Parser0 which may parse nothing, so an inclusive range could fail there, but then again, maybe that is a feature? Maybe you only want the Span of a Parser?

[codecov-commenter]

Codecov Report

Merging #301 (bcff7b1) into main (714a9df) will increase coverage by 0.21%.
The diff coverage is 97.22%.

@@            Coverage Diff             @@
##             main     #301      +/-   ##
==========================================
+ Coverage   96.43%   96.65%   +0.21%     
==========================================
  Files           8        9       +1     
  Lines        1011     1404     +393     
  Branches       81      133      +52     
==========================================
+ Hits          975     1357     +382     
- Misses         36       47      +11     

Impacted Files	Coverage Δ
core/shared/src/main/scala/cats/parse/Parser.scala	96.47% <90.00%> (+0.15%)	⬆️
core/shared/src/main/scala/cats/parse/Caret.scala	100.00% <100.00%> (ø)
...shared/src/main/scala/cats/parse/LocationMap.scala	100.00% <100.00%> (+2.94%)	⬆️

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[regadas]

🎨 this condition is popping a few times would a isValidOffset be worth?

[johnynek]

good idea. I've made that change (and avoid checking it so many times).