fix(encoder): skip indentation for top-level sequences

[nervo]

When using encoder IndentSequence option, top level sequences are always indented, like i've already spotted in #291 (4 years ago, omg :) )
The playground is still available here https://go.dev/play/p/_mn80-HkPHK

Anyway, after digging into the code, i have found that indentation is handled differently for maps and sequences:

• directly into encodeSlice/encodeArray for slices, using e.column, without any solution to check the top level
• into encodeMapItem/encodeMap/encodeStruct for maps, by applying reverse logic, aka. considering that indentation is only appliable for embedded maps.

So i have just moved the sequence indentation logic at the just same level of maps, add some tests, and voila :)

Fix #291

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.49%. Comparing base (d4021ff) to head (5df9c7b).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #855      +/-   ##
==========================================
- Coverage   80.51%   80.49%   -0.03%     
==========================================
  Files          22       22              
  Lines        6847     6848       +1     
==========================================
- Hits         5513     5512       -1     
- Misses        913      914       +1     
- Partials      421      422       +1     

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