Core: Delete implicit Variant comparison by default

[Repiteo]

Revamps the Variant comparison operators to:

• Utilize all comparison operations, with < and == as the baseline.
• Add deleted template comparison operators behind STRICT_CHECKS macros

In practice, this makes all Variant comparison operations entirely self-contained, necessitating they be called explicitly. We already had to do this with certain ambiguous types (arithmetic types most obviously), so this is just enforcing that for all other types. It also helps showcase some areas with questionable Variant arguments/returns, that were otherwise being quietly converted. Keeping it behind the STRICT_CHECKS macro means that we aren't breaking compatibility out of the gate, so modules/extensions can gradually adopt this convention.