Add missing copy!(::AbstractMatrix, ::UniformScaling) method (#55970)

Hi everyone! First PR to Julia here.

It was noticed in a Slack thread yesterday
that `copy!(A, I)` doesn't work, but `copyto!(A, I)` does. This PR adds
the missing method for `copy!(::AbstractMatrix, ::UniformScaling)`,
which simply defers to `copyto!`, and corresponding tests.

I added a `compat` notice for Julia 1.12.

---------

Co-authored-by: Lilith Orion Hafner <[email protected]>

Author: dzhang314

stdlib/LinearAlgebra/src/uniformscaling.jl

@@ -403,6 +403,16 @@ function copyto!(A::Tridiagonal, J::UniformScaling)
     return A
 end
 
+"""
+    copy!(dest::AbstractMatrix, src::UniformScaling)
+
+Copies a [`UniformScaling`](@ref) onto a matrix.
+
+!!! compat "Julia 1.12"
+    This method is available as of Julia 1.12.
+"""
+Base.copy!(A::AbstractMatrix, J::UniformScaling) = copyto!(A, J)
+
 function cond(J::UniformScaling{T}) where T
     onereal = inv(one(real(J.λ)))
     return J.λ ≠ zero(T) ? onereal : oftype(onereal, Inf)

stdlib/LinearAlgebra/test/uniformscaling.jl

@@ -226,6 +226,13 @@ let
         @test copyto!(B, J) == [λ zero(λ)]
     end
 
+    @testset "copy!" begin
+        A = Matrix{Int}(undef, (3,3))
+        @test copy!(A, I) == one(A)
+        B = Matrix{ComplexF64}(undef, (1,2))
+        @test copy!(B, J) == [λ zero(λ)]
+    end
+
     @testset "binary ops with vectors" begin
         v = complex.(randn(3), randn(3))
         # As shown in #20423@GitHub, vector acts like x1 matrix when participating in linear algebra