refactor: introduce SinglyLinkedListNode

[vil02]

This is a step towards not suppressing auxiliaryclass:

Java/pom.xml

Line 78 in 22098c7

<arg>-Xlint:-auxiliaryclass</arg>

This PR moves the Node class outside SinglyLinkedList.java.
The name SinglyLinkedListNode has the advantage that it is unique. I am open for suggestions here.

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

❌ Patch coverage is 92.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.77%. Comparing base (22098c7) to head (4e38765).
⚠️ Report is 450 commits behind head on master.

Files with missing lines	Patch %	Lines
...orithms/datastructures/lists/SinglyLinkedList.java	83.33%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6210      +/-   ##
============================================
- Coverage     73.78%   73.77%   -0.01%     
- Complexity     5296     5299       +3     
============================================
  Files           670      671       +1     
  Lines         18343    18344       +1     
  Branches       3546     3546              
============================================
  Hits          13534    13534              
- Misses         4262     4263       +1     
  Partials        547      547              

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[siriak]

Looks good, thanks!