update unicode "₊" separator to use the default separator variable

[isaacsas]

SciML/ModelingToolkit.jl#2798

We need to propagate this throughout Catalyst where needed (probably some tutorials, tests, and the symmap_to_varmap code). Also need to update to depend on MTK 9.18 or later only.

[ChrisRackauckas]

#951

[isaacsas]

There are still some tests and docs that explicitly use "₊". We need to update them whenever MTK decides to change the separator.

[ChrisRackauckas]

Just use the general separator const?

[isaacsas]

Yup that is all that needs to be done for the tests, but the doc example will require a bit more tweaking.