CI + docs refresh to new julia versions

[c42f]

• Update CI to run on recent Julia versions
• Updated Documenter version + jldoctest blocks for Julia 1.6

[codecov-commenter]

Codecov Report

Merging #23 (1ad0834) into master (384faa7) will increase coverage by 0.47%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #23      +/-   ##
==========================================
+ Coverage   96.29%   96.77%   +0.47%     
==========================================
  Files           1        1              
  Lines          54       62       +8     
==========================================
+ Hits           52       60       +8     
  Misses          2        2              

Impacted Files	Coverage Δ
src/Underscores.jl	96.77% <ø> (+0.47%)	⬆️

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