Add `CrosswordSolver.java` new algorithm

[Hardvan]

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

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.93939% with 2 lines in your changes missing coverage. Please review.

Project coverage is 58.39%. Comparing base (435532f) to head (b2b6de4).

Files with missing lines	Patch %	Lines
...om/thealgorithms/backtracking/CrosswordSolver.java	93.93%	0 Missing and 2 partials :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5626      +/-   ##
============================================
+ Coverage     58.31%   58.39%   +0.08%     
- Complexity     3714     3733      +19     
============================================
  Files           552      553       +1     
  Lines         15906    15939      +33     
  Branches       3022     3034      +12     
============================================
+ Hits           9275     9307      +32     
+ Misses         6235     6234       -1     
- Partials        396      398       +2     

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