Missing promotions in Tridiagonal and SymTridiagonal constructors

[stevengj]

This is a suboptimal experience:

julia> using LinearAlgebra

julia> SymTridiagonal([1,2,3],[4,5.0])
ERROR: MethodError: no method matching SymTridiagonal(::Vector{Int64}, ::Vector{Float64})

Should be as easy as adding a constructor that does promotion?

[putianyi889]

LazyBandedMatrices.Tridiagonal could be a temporary workaround.