__traits for checking if a type is dynamic array (slice)

[Per Nordlöw]

Why is there no

- __traits(isArray, T)

alongside

- __traits(isStaticArray, T) and
- __traits(isAssociativeArray, T)

when dmd already has `ENUMTY.Tarray` alongside

- ENUMTY.Tsarray and
- ENUMTY.Taarray

and std.traits already has a wrapper for this at

https://dlang.org/phobos/std_traits.html#isDynamicArray

?

Should we add this new builtin trait and use it in 
std.traits.isDynamicArray?

If so, should we call it

- __traits(isDynamicArray) or
- __traits(isArray) or
- __traits(isArraySlice) or
- __traits(isSlice) or

something else?