use iszero in isinteger(::AbstractFloat) (#55276)
This is a very slight tweak to the implementation of
`isinteger(::AbstractFloat)` to use `iszero` rather than `== 0`. It
shouldn't make any difference with any of the built-in floating-point
types, but `iszero` might conceivably be faster for some user-defined
types.
I also added a comment to indicate why it's using `iszero(x - trunc(x))`
rather than `x == trunc(x)` (due to non-finite values); this code dates
back to #14569 in Julia 0.5.
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Co-authored-by: Sukera <11753998+Seelengrab@users.noreply.github.com>
