Add arbitraries for numeric string

[matwojcik]

No description provided.

[codecov]

Codecov Report

Merging #918 (f905cf8) into master (b9b027b) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #918      +/-   ##
==========================================
+ Coverage   91.63%   91.75%   +0.12%     
==========================================
  Files          49       49              
  Lines         538      546       +8     
  Branches       12       11       -1     
==========================================
+ Hits          493      501       +8     
  Misses         45       45              

Impacted Files	Coverage Δ
...n/scala/eu/timepit/refined/scalacheck/string.scala	100.00% <100.00%> (ø)

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[fthomas]

LGTM. Many thanks @matwojcik!