adds json-ld serialization

[thomas-topway-it]

adds json-ld serialization using https://github.com/easyrdf/easyrdf
from SMW's standard RDF serialization

for testing purpose run composer install in the extension folder after merging the pull request #79

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 42 lines in your changes are missing coverage. Please review.

Project coverage is 79.20%. Comparing base (4f1cce3) to head (2946b91).

Files	Patch %	Lines
src/JsonLDSerializer.php	0.00%	39 Missing ⚠️
src/HookRegistry.php	0.00%	3 Missing ⚠️

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Additional details and impacted files

@@              Coverage Diff              @@
##             master      #80       +/-   ##
=============================================
- Coverage     97.28%   79.20%   -18.08%     
- Complexity       67       80       +13     
=============================================
  Files             6        7        +1     
  Lines           184      226       +42     
=============================================
  Hits            179      179               
- Misses            5       47       +42     

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