feat: support ignored value "-" for env tag

[sv-kozlov]

Support for ignoring a field when reading struct.
This is similar to using json:"-".
If the env tag of a field has the value "-", the field is always ignored.

Note, in the env tag, the value "-" can be set not only in place of the name, but also in the list of options.
This is done on the assumption of development convenience, so as not to remove the entire contents of the env tag.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (1cb1967) to head (e213ff6).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #338   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            3         3           
  Lines          524       529    +5     
=========================================
+ Hits           524       529    +5     

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[caarlos0]

thanks!