Convert to slnx

[bkoelman]

Convert sources to use .slnx solution file format.

Closes #1912.

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• N/A: Adapted tests
• N/A: Documentation updated

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.19%. Comparing base (453aa2a) to head (28748e5).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1923   +/-   ##
=======================================
  Coverage   92.19%   92.19%           
=======================================
  Files         437      437           
  Lines       14883    14883           
  Branches     2464     2464           
=======================================
  Hits        13722    13722           
  Misses        714      714           
  Partials      447      447           

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