Tighten signature of iszero and isone fallbacks

[lgoettgens]

Resolves #57372. Please see that issue for the motivation behind the change.

Could someone please run a PkgEval on this? If there are breakages in the ecosystem, I think this is a sign to not follow down this any further.

[nsajko]

This PR currently breaks at least the following functionality:

• linear algebra
• dates
• TwicePrecision (used for floating-point ranges)

[jishnub]

This seems to be quite breaking for somewhat limited gains. A lot of the breakage might happen in the wild, and wouldn't be caught by a PkgEvalrun.

I'm also not fully certain about the motivation here. One of the examples in the issue is iszero(Int) returns false, but probably a specific method should be added for Types to avoid such comparisons? Also, probably a similar method may be added for Ring? After all, iszero(x) is documented to be a check for x == zero(x), where zero(x) is the additive identity. Special cases should generally be added when this isn't what zero returns.