Stricter type checking for strings in arithmetic operators

[sastanin]

Recently I encountered a bug in the model which was due to this Flexscript feature:

`string` values can be used in arithmetic expressions, and there are no warnings, exceptions, compilation errors, anything. `x` is silently converted to a numeric value zero. To make matter worse, integer 3 and string "3" are displayed identically in the debugger, and `x` can be a Variant.

While some programming languages (like Javascript, Powershell) do permit such code, they parse strings as numbers. The others do not permit such expressions without explicit type conversions.

I think it would be a good idea to make Flexscript type checker slightly stricter and require an explicit .toNum() conversion (or go the Javascript way). I understand that this is a breaking change, but otherwise there is absolutely no way to know if arithmetic expressions are evaluated correctly.

Relevant in FlexSim up to 21.2 Beta.