Add AbstractAlgebra.is_ijulia_inited and an IJulia extension driving it

[fingolfin]

Co-authored-by: Codex codex@openai.com

Resolves #2357

[codecov]

Codecov Report

❌ Patch coverage is 0% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.19%. Comparing base (185b6ce) to head (c2f6354).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
src/AbstractAlgebra.jl	0.00%	5 Missing ⚠️
src/PrettyPrinting.jl	0.00%	4 Missing ⚠️
ext/IJuliaExt/IJuliaExt.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2365      +/-   ##
==========================================
- Coverage   88.20%   88.19%   -0.01%     
==========================================
  Files         127      128       +1     
  Lines       32865    32855      -10     
==========================================
- Hits        28987    28978       -9     
+ Misses       3878     3877       -1     

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[fingolfin]

I've now also updated the outdated code comment (or rather I asked codex to update it)

[fingolfin]

I note that Hecke and Oscar contain similar hacks. Two solutions come to mind:

1. let them all use AbstractAlgebra.is_ijulia_inited() (then they need to depend on AA in a suitable version)
2. let them all add their own package extension with equivalent code (means it can be done immediately, without an AA release).

I tend towards option 1. Any thoughts, @lgoettgens @thofma ? And, shall we merge this?

[thofma]

I vote for (1).

[lgoettgens]

I am also in favor of (1). We should do another release soon-ish anyway to make #2362 available to downstream users.