Declare `LinearAlgebra.AbstractTriangular` to be public (#52246)
Since methods such as `mul!`, `(l/r)mul!` and `(l/r)div!` are defined
for `AbstractTriangular`, having this be public allows easier
disambiguation in packages. [Several
packages](https://juliahub.com/ui/Search?q=AbstractTriangular&type=code)
are already using it as if it's public, so it makes sense to support
this.
This would resolve the specific issue in
https://discourse.julialang.org/t/methoderrors-suggest-fixes-that-may-depend-on-internal-bindings/106468
