Add c++20 kernel

[mcbarton]

The purpose of this PR is to add a c++20 kernel.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.42%. Comparing base (d203fdf) to head (dd506c4).
⚠️ Report is 240 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #77      +/-   ##
==========================================
+ Coverage   51.60%   52.42%   +0.81%     
==========================================
  Files          15       15              
  Lines         593      599       +6     
==========================================
+ Hits          306      314       +8     
+ Misses        287      285       -2     

see 1 file with indirect coverage changes
see 1 file with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[vgvassilev]

I am not sure I understand how this works. We need to pass somehow -fopenmp as a flag. XEUS_CPP_OMP probably does not expand to anything...

[mcbarton]

I'll remove adding the omp-17 kernel and move that to a different PR. I assumed that infrastructure was there to use it, but had accidentally been missed. I'll leave this PR to just adding the c++20 kernel.

[mcbarton]

I'll remove adding the omp-17 kernel and move that to a different PR. I assumed that infrastructure was there to use it, but had accidentally been missed. I'll leave this PR to just adding the c++20 kernel.

@vgvassilev I've made this change now, so the PR should hopefully be ok to merge now.

[vgvassilev]

LGTM. I'd wait for @JohanMabille.