Fix warnings on PHP 8.1

[jtojnar]

With, PHP 8.1 lack of a return type hint for a method of a child class, whose parent has return type hint is considered mismatch.

https://php.watch/versions/8.1/internal-method-return-types#no-return-type

Since standard classes and interfaces started adding typehings, errors like the following are produced:

PHP Deprecated: Return type of Violet\StreamingJsonEncoder\AbstractJsonEncoder::current() should either be compatible with Iterator::current(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

void type is only available since PHP 7.1 and mixed type requires PHP 8.0:

https://www.php.net/manual/en/language.types.declarations.php#language.types.declarations.void https://www.php.net/manual/en/language.types.declarations.php#language.types.declarations.mixed

To preserve compatibility with PHP < 8.0, we need to add the attribute.

[codecov-commenter]

Codecov Report

Merging #4 (21fdb9f) into master (31661f6) will not change coverage. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##              master        #4   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity       114       114           
===========================================
  Files              4         4           
  Lines            296       296           
===========================================
  Hits             296       296           

Impacted Files	Coverage Δ
src/AbstractJsonEncoder.php	100.00% <100.00%> (ø)

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[Riimu]

Thanks, I merged this via #6, since I needed to rebase it.