ref: Move raw dataset code to litdata/raw, expose StreamingRawDataset at top-level

[bhimrazy]

What does this PR do ?

This PR reorganizes all raw dataset-related files into the new litdata/raw directory and moves corresponding tests to tests/raw.

Also, exposes StreamingRawDataset in the top-level litdata import for easier access.

No functional changes; only refactoring and import improvements.

Follow up to #652

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 84%. Comparing base (5a57d85) to head (23b6034).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #671   +/-   ##
===================================
  Coverage    84%    84%           
===================================
  Files        50     52    +2     
  Lines      7137   7143    +6     
===================================
+ Hits       5960   5967    +7     
+ Misses     1177   1176    -1     

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