`tril` and `triu` not working with static arrays

At the moment tril and triu do not work for static arrays

julia> B = @SMatrix [1 2 3 4;5 6 7 8;9 10 11 12; 13 14 15 16]
4×4 SMatrix{4, 4, Int64, 16} with indices SOneTo(4)×SOneTo(4):
  1   2   3   4
  5   6   7   8
  9  10  11  12
 13  14  15  16
julia> tril(B)
ERROR: setindex!(::SMatrix{4, 4, Int64, 16}, value, ::Int) is not defined.
Stacktrace:
 [1] error(s::String)
   @ Base .\error.jl:33
 [2] setindex!(a::SMatrix{4, 4, Int64, 16}, value::Int64, i::Int64)
   @ StaticArrays ~\.julia\packages\StaticArrays\rdb0l\src\indexing.jl:3
 [3] macro expansion
   @ ~\.julia\packages\StaticArrays\rdb0l\src\indexing.jl:66 [inlined]
 [4] _setindex!_scalar
   @ ~\.julia\packages\StaticArrays\rdb0l\src\indexing.jl:46 [inlined]
 [5] setindex!
   @ ~\.julia\packages\StaticArrays\rdb0l\src\indexing.jl:42 [inlined]
 [6] tril!
   @ C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\LinearAlgebra\src\dense.jl:176 [inlined]
 [7] tril!
   @ C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\LinearAlgebra\src\generic.jl:443 [inlined]
 [8] tril(M::SMatrix{4, 4, Int64, 16})
   @ LinearAlgebra C:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.6\LinearAlgebra\src\generic.jl:365
 [9] top-level scope
   @ REPL[126]:1

I think it might be useful in some situations to have those working for static arrays.

Start by reproducing the reported REPL example for tril(B), then inspect StaticArrays' indexing.jl alongside the LinearAlgebra tril! and triu! entry points shown in the stack trace. Done means both tril and triu operate on static arrays without the reported setindex! error, with behavior checked for the example.