Support Seq and List decoding

[geirolz]

Fix #137 in order to provide a proper Decoder[Seq[T]] and Decoder[List[T]] implementation.

Moreover flat groups in order to support Down operator in nested cases

[codecov]

Codecov Report

Attention: 13 lines in your changes are missing coverage. Please review.

Comparison is base (843a291) 65.11% compared to head (06bad04) 65.24%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #138      +/-   ##
==========================================
+ Coverage   65.11%   65.24%   +0.12%     
==========================================
  Files          59       59              
  Lines        2680     2690      +10     
  Branches      143      303     +160     
==========================================
+ Hits         1745     1755      +10     
  Misses        935      935              

Files	Coverage Δ
core/src/main/scala/cats/xml/xmlNode.scala	69.04% <100.00%> (+1.23%)	⬆️
...src/main/scala/cats/xml/std/nodeSeqConverter.scala	71.42% <66.66%> (ø)
core/src/main/scala/cats/xml/NodeContent.scala	74.41% <62.50%> (-10.59%)	⬇️
core/src/main/scala/cats/xml/codec/Decoder.scala	76.63% <45.45%> (+1.27%)	⬆️

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[github-advanced-security]

Codacy-scalameta-pro (reported by Codacy) found more than 10 potential problems in the proposed changes. Check the Files changed tab for more details.

[github-advanced-security]

Scalastyle (reported by Codacy) found more than 10 potential problems in the proposed changes. Check the Files changed tab for more details.