Add a lint to ban num

[a14n]

From #57779

maybe a better lint to address my concern is one to ban num entirely (whether given explicitly or by inference).

[a14n]

@Hixie what would you put in lint's description of this lint to explain why num should be ban ?

[Hixie]

The primary idea is to avoid having code paths where the compiler needs to handle either int or double. cc @mraleph @a-siva @aam who might be able to confirm whether it's indeed more efficient to have code that unambiguously uses always int or always double or whether actually using num isn't such a big deal.

(The secondary reason here is to keep the code clear and catch likely bugs. It's often the case that if your code involves mixing int and double, you've probably stumbled into a less-than-fully-designed case that needs more thought.)

[mraleph]

For computational kernels it is indeed better to have code that uses either int or double not num.

TFA sometimes might be able to infer what is stored in a particular num typed location - but not always.