Support other names for envDefault

[dnovikoff]

We can change the "env" to some other tag name using opt.TagName. I think that it will be more consistent to have an opt.DefaultTagName to change the tag for "envDefault".

That could help switching from other env libraries.

[ccoVeille]

I like your PR as it could help to keep tag in the struct in our prefered case (snake_case, kebab, … whatever)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (aa50469) to head (a3b3e46).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #324   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines          499       504    +5     
=========================================
+ Hits           499       504    +5     

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[caarlos0]

thanks!