Replace AbstractTriangular by UpperOrLowerTriangular

[dkarrasch]

This is in preparation of #26307. Especially when type parameters are used, I believe that the union of the four *Triangular types is actually meant.

For simplicity, I have copied the aliases from current LinearAlgebra to avoid VERSION-dependent code. I could add a note that these definitions could be removed once the lower compat is raised to v1.10, if desired.

[codecov]

Codecov Report

Merging #217 (ca85f94) into master (3c12489) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #217   +/-   ##
=======================================
  Coverage   99.08%   99.08%           
=======================================
  Files           4        4           
  Lines         656      656           
=======================================
  Hits          650      650           
  Misses          6        6           

Impacted Files	Coverage Δ
src/FillArrays.jl	99.69% <100.00%> (ø)

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[dkarrasch]

That's strange. On master, checksquare is not imported, and not used in my PR. How come it gets added when you merge master into this branch? It doesn't matter, of course, but still strange.

[jishnub]

checksquare is imported on master in 3c12489

[dkarrasch]

I see, I missed the revert-and-resubmission.