feat: add Neo4j graph backend

[gusye1234]

• Add Neo4j graph backend
• Rearrange _storage to folder
• Some styling for dspy extraction

Known issues for this PR:

• Didn't find a suitable workflow action for neo4j right now, so the test of neo4j storage is skipped for now.
• Leiden in neo4j is not stable, even given the same rand seed

[codecov]

Codecov Report

Attention: Patch coverage is 95.93909% with 8 lines in your changes missing coverage. Please review.

Project coverage is 94.13%. Comparing base (b33b2b8) to head (ba04e76).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
nano_graphrag/_utils.py	57.14%	3 Missing ⚠️
nano_graphrag/_storage/kv_json.py	92.00%	2 Missing ⚠️
nano_graphrag/graphrag.py	77.77%	2 Missing ⚠️
nano_graphrag/_storage/vdb_nanovectordb.py	97.14%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #61      +/-   ##
==========================================
+ Coverage   94.03%   94.13%   +0.10%     
==========================================
  Files          12       16       +4     
  Lines        1290     1330      +40     
==========================================
+ Hits         1213     1252      +39     
- Misses         77       78       +1     

Flag	Coverage Δ
	94.13% <95.93%> (+0.10%)	⬆️

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