Implement Trixi.Threaded()

[vchuravy]

Implements #2997 (comment)

I am not happy with the name Threaded since it is the one "owned" by FastBroadcast.jl,
but I am also too tired to come up with a better name.

  1. FastBroadcast.Threaded already exists as a type with the same name, so users might conflate the two                                    
  2. The original comment in the PR called the Parallel() name "bad" — Threaded() is arguably worse for the same reason                     
                                                                                                                                            
  Some alternatives to consider:                                                                                                            
                                                                                                                                            
  - thread_preference() — describes what it does (returns the thread kwarg value based on preference)                                       
  - ode_thread_arg() — explicit that it's for ODE solver thread= kwargs                                                                     
  - Trixi.thread() — short, mirrors the kwarg name thread = Trixi.thread()                                                                  
  - preferred_thread() — communicates it reads the preference                                                                               
                                                                                                                                            
  My suggestion would be thread_preference() since it makes it clear the value is derived from Trixi's configured preference. Or simply     
  thread() for terseness, which reads naturally as RDPK3SpFSAL49(thread = Trixi.thread()).

Replaces #2997

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.12%. Comparing base (bb36217) to head (0f0c329).

Files with missing lines	Patch %	Lines
src/Trixi.jl	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3003      +/-   ##
==========================================
- Coverage   97.13%   97.12%   -0.01%     
==========================================
  Files         625      625              
  Lines       48514    48518       +4     
==========================================
+ Hits        47122    47123       +1     
- Misses       1392     1395       +3     

Flag	Coverage Δ
unittests	97.12% <25.00%> (-0.01%)	⬇️

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[ranocha]

My suggestion would be thread_preference()

Why did you not follow "your" suggestion?

[ranocha]

Shouldn't we check whether Trixi.jl threading uses Polyester.jl instead? I would expect to get problem when using Base threads in Trixi.jl and Polyester.jl threads in the time integrator.