`SymmetricTensor` not working nicely with Symbolics.jl

[henrij22]

I was working with the Symbolics.jl package recently and it seems mostly to work out of the box.
However converting a Tensor to a SymmetricTensor leads to an error:

A symbolic expression appeared in a Boolean context. This error arises in situations where Julia expects a Bool, like
if boolean_condition             use ifelse(boolean_condition, then branch, else branch)
x && y                           use x & y
boolean_condition ? a : b        use ifelse(boolean_condition, a, b)
but a symbolic expression appeared instead of a Bool. For help regarding control flow with symbolic variables, see https://docs.sciml.ai/ModelingToolkit/dev/basics/FAQ/#How-do-I-handle-if-statements-in-my-symbolic-forms?

However that can be easily fixed with

using LinearAlgebra
using Symbolics

LinearAlgebra.issymmetric(t::Tensor{2, 2, Num}) = @inbounds t[1,2] === t[2,1]

(Note the three =).

If you are interested I could draft up a PR putting this code into an extension, marking it only to be used when Symbolics.jl is also used. If you think that is too user-specific its also fine.